Per @timneutkens suggestion [here](https://github.com/zeit/next.js/pull/5631#issuecomment-440588455) I went ahead and gave a shot at trying to document `renderPage` usage.

This PR:
- Documents `renderPage` legacy and current usage.

* Added a new example with relay modern and a graphql server with express.

* removed .graphqlconfig file from with-relay-modern-server-express example

* ADD with-firebase-hosting-and-docker example

* Improve doc

This PR Fixes #4920

So the problem is that when a next.js application is built on windows, the `pages-manifest.json` file is created with backslashes. If this built application is deployed to a linux hosting enviroment, the server will fail when trying to load the modules.

```
Error: Cannot find module '/user_code/next/server/bundles\pages\index.js
```

My simple solution is to modify the `pages-manifest.json` to always use linux separator (`/`), then also
modify `server/require.js` to, when requiring page, replace any separator (`\` or `/`) with current platform-specific file separator (`require('path').sep`).

The fix in `server/require.js` would be sufficient, but my opinion is that having some cross-platform consistency is nice.

This change was tested by bulding an application in windows and running it in linux and windows, aswell as building an application in linux and running it in linux and windows. The related tests was also run.
# Conflicts:
#	test/integration/production/test/index.test.js

The demo did not work. Updating to babel-plugin-inline-react-svg v1.0.1 did work.

Google seems to be deprecated the legacy realtime database and moving towards default use of Firestore, although it's still officially in beta. This PR migrates towards Firestore and the recommended loading methods for the Firebase 5.6.0 libraries. Note: the Firebase and Firebase-Admin dependencies should be updated to 5.6.0 and 6.3.0 respectively.

Hey @timneutkens I've updated this example a bit.

- Fixed `/asserts` spelling to `/assets`.
- Removed the `/assets/styles.less` as importing this caused the entire ant-design css sheet to be loaded, now only the React components imported will have their styles loaded via the babel plugin which was already configured. Resulted in dropping the CSS for this example from ~630kb to ~220kb.
- Removed `index.js` as it's not needed.

* Remove app variable as it’s only used once

* Remove double spread

* Move send-html function and rewrite in typescript

* Move getPageFiles and convert to ts

* Move getPageFiles and convert to ts (#5841)

* Move getPageFiles and convert to ts

# Conflicts:
#	packages/next-server/server/render.js

* Fix unit tests

* Move getPageFiles and convert to ts

# Conflicts:
#	packages/next-server/server/render.js

* Fix unit tests

* Added length calculation for insert of cache rather then default to 1. changed default cache from 100 to 100MB

* adjusted for lint

Couldn't find a reference to these functions as next-server uses the renderToHTML directly.

We don't have to check if the file already exists here, since it's always in production mode (dev overrides the readBuildId method to always be `development`) If the file is not found (error is thrown) we check if the file exists. If not we throw a helpful error. In other cases we throw the original error.

Firebase has some issues with grpc. ( Firebase has some issues with grpc ) This is resolved in a newer version of firebase. Especially with npm 6.4.1. https://github.com/firebase/firebase-js-sdk/issues/1341

Issue #5688

* Add tsc type checking

* Add linting on circle

* Add node-fetch types

* Use strict mode

We ran into this eg on hyper-site, which has `module.exports` in the content.

* add new example

* update gitignore

* fix lint

* fix linting

* fix linting again

* update unstated example

* remove unsued var

* update readme

* Add docs for returning `null` from generateBuildId

* Add test for setting custom buildid

* Fix linting

* Delete .babelrc

* Update package.json

* Create next.config.js

* Update next.config.js

#5521

There were some strange quotes and misaligned html on the page in the `with-emotion` example.