Building off [this Slack conversation](https://vercel.slack.com/archives/CGD3XGSD7/p1597329727013900), this PR adds a top-level section to the documentation on authentication patterns.

Please provide any and all comments! A few open thoughts I have:

- ~Should this include code snippets from the related providers or stay very high-level? At what point do we delegate to the examples folder?~ Keep things high level and delegate to examples folder 
- ~Should this include any related cards at the bottom?~ Added to the bottom
- ~Should other places in the documentation link back to here?~ Added link from routing
- Should it be a top-level route, or be underneath advanced?

The HTML Living Standard explicitly says `href` should be omitted to prevent the loading of an incorrectly sized image:
https://html.spec.whatwg.org/multipage/semantics.html#attr-link-imagesrcset

![image](https://user-images.githubusercontent.com/616428/103378205-8a013800-4aaf-11eb-9085-10f547263fed.png)

Since it's in the spec, I assume this is valid-enough HTML.

This also dedupes preloads which the old implementation did not.

---

Fixes #18756
x-ref #19118
Fixes #18720

Closes https://github.com/vercel/next.js/issues/20036.

```
images: {
    domains: ['example.com'],
    path: 'https://example.com/myaccount/',
},
```
Those `domains` and `path` look a lot alike for me and so, I was confused. I found out that the domains are ignored if the Loader is set, which makes sense.

I don't think the docs should still mention the `priority` prop because the feature no longer exists since this was merged -  https://github.com/vercel/next.js/pull/19118

It certainly caught me out! 😬 https://github.com/vercel/next.js/issues/20438

* Docs typo fix

* Update docs/basic-features/environment-variables.md
Co-authored-by: NLachlan Campbell <lachlanjc@hey.com>
Co-authored-by: NJoe Haddad <timer150@gmail.com>
Co-authored-by: NLachlan Campbell <lachlanjc@hey.com>
Co-authored-by: Nkodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

Upgraded all `http` links to `https` that support the `https` protocol.

Llinks
-----
- https://github.com/
- https://koajs.com
- https://ant.design
- https://www.carbondesignsystem.com/components/overview
- https://themes.carbondesignsystem.com
- https://knexjs.org
- https://jxnblk.com/rebass
- https://i.imgur.com/JCxtWSj.gif
- https://react.semantic-ui.com
- https://stomp.github.io
- https://videojs.com

Co-authored-by: NLuis Alvarez D. <luis@vercel.com>
Co-authored-by: Nkodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

Changed reference from 'master' to 'main' branch under DPS section in line with GitHub's recent switch from master to main branch as base repo branch names.

This should fix  source map config description text.

Makes https://www.npmjs.com/package/@zeit/next-source-maps a built-in feature so that it can be deprecated.

This PR is an implementation of RFC #13276 (more specifically @banerjeesouvik's [suggestion](https://github.com/vercel/next.js/discussions/13276#discussioncomment-17874)) which solves #3322 by adding an extra parameter to the router events that'll allow you to see whether or not the route is shallow.

This is a recreation of PR #13243.

Mention onClick property for next/link wrapping custom components

Add note about import required to be explicit and dynamic required to be top-level module.

Grammar correction.

Currently if sizes is not defined, Next.js is setting sizes as:
```
(max-width: 640px) 640px, (max-width: 750px) 750px, (max-width: 828px) 828px, (max-width: 1080px) 1080px, (max-width: 1200px) 1200px, (max-width: 1920px) 1920px, (max-width: 2048px) 2048px, 3840px'
```

This pull request will make sizes be `100vw` by default, which will allow us to download "smaller" images than what's currently happening. 

In a demo app I have, the difference is between downloading 488KB vs 1.4MB (in images)

Closes: https://github.com/vercel/next.js/issues/20044

This makes sure to note how custom-routes need to be prefixed when `locale: false` is used for routes to be matched correctly. 

Closes: https://github.com/vercel/next.js/issues/19703

Remove extra word "the"

We've received some feedback that the current note about calling API routes inside gSP/gSSP is confusing. This updates the wording to make it clear you can still use `fetch` in your application, and also to not say you "import" an API route. You import the _logic_ inside the route.

dotenv currently doesn't support inline comments (see https://github.com/motdotla/dotenv/issues/484), so it's misleading to use inline comments in documentation examples.

Either use `avoid` or `prevent` as verb, but not both.

To clarify what I've noticed as a common source of confusion in discussions online. Fixes https://github.com/vercel/next.js/issues/19426 

If you wish to know whether the code is running as part of a client-side page transition, in either `getInitialProps` or `getServerSideProps`, you can check to see if `context.req.url` starts with `/_next/data`.

Fixes https://github.com/vercel/next.js/issues/19503

- Updated some links in docs that don't have the `.md` extension. Not required for our live docs but useful for the GitHub view.
- Updated links that go to https://nextjs.org/docs to a relative path, that way they'll work for versioned docs
- Updated the `Regex Path Matching` example to be consistent with the paragraph above and with the official example in /examples.

For an API route to work, you need to `export as default a function` (a.k.a **request handler**), which then receives the following parameters:
For an API route to work, you need to `export a function as default` (a.k.a **request handler**), which then receives the following parameters:


I might be wrong though (not a native English speaker), it just sounds strange to my ears.

Fixes https://github.com/vercel/next.js/issues/19486

Added the line _Prefetch can be disabled by passing `prefetch={false}`._

This adds a note about current `i18n` support and `next export` since it is not currently supported as was mentioned in the [initial RFC](https://github.com/vercel/next.js/discussions/17078)

x-ref: https://github.com/vercel/next.js/issues/18318

Some slight tweaks to match the corresponding blog post.

Co-authored-by: Nkodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

Building off the "Migrating to Next.js" section started, this doc provides information on using `rewrites`, `redirects`, and micro-frontends to incrementally adopt Next.js in your codebase.

ref. https://github.com/vercel/next.js/pull/14519, https://github.com/vercel/next.js/pull/15062
Add limitation for Anonymous arrow functions.

~Updated the `with-zones` example and docs to use `@vercel/next` instead of `@now/next` and also updated the readme instructions with better steps.~

~Related to https://github.com/vercel/next.js/issues/12317 - Looks like there may be an issue with the example but it continues to work as expected to me, moving away from `@now/next` may help.~

Fixes https://github.com/vercel/next.js/issues/12317
Closes https://github.com/vercel/next.js/issues/12317

Moved the example to use [Vercel Monorepos](https://vercel.com/blog/monorepos). Locally it switches to `next dev` instead of `vercel dev`.

Update the mentioned redirection status message.

This mirrors the `basePath: false` behavior for custom-routes with `locale: false` to allow users to configure the locales manually (`locale: false`) or have the locales be handled automatically for custom-routes. 

Fixes: https://github.com/vercel/next.js/issues/18927
Fixes: https://github.com/vercel/next.js/issues/18795