Co-authored-by: NJoe Haddad <joe.haddad@zeit.co>

This updates old examples to the more universal `npx` command.

Fixes https://github.com/vercel/next.js/discussions/12103

Co-authored-by: NJoe Haddad <joe.haddad@zeit.co>

* npx create → npm init

* Fix inconsistent instructions

* Update amp-first

* Update with-graphql-react

* with-firebase-cloud-messaging

* Update with-higher-order-component

* change create-next-app url

* Update create-next-app instruction

* Update instructions to use npm instead of npx

* Move "the idea behind the example" to top

* Rename

* Rename

* Update contributing.md with a README template

* Remove Deploy To Now buttons

* Remove remaining buttons

Adding an example of testing with Mocha

#4767

Preferably this installation wouldn't be necessary, but in lieu of a fix...

#4751

* Examples: clarify language around Yarn create & npx

* add missing READMEs and create-next-app usage

* suggest people tag jthegedus in firebase related issues

* add yarn alt instructions

* cerebraljs example readme & fixes

* remove global npm install of create-next-app

* add npx to create-next-app command in examples

* add bash to shell snippets

* add yarn create to next-app command in examples

* fix READMEs named with lowercase

* change READMEs to use UPPERCASE

* examples: add create-next-app

* fix with-typescript readme

Closes #2895

* Add deploy now button to the top of all example READMEs

* Remove unnecessary whitespace changes

* Fix link

* Fix links

* Added data fetch example

* Finished updating examples install instructions and added data-fetch example to main readme

* Added missing readme and references in main readme.

* Fixed wrong links

* Updated examples format

* Added redux and styled components (wip) examples.

* Updated examples readmes and package.json

* Fixed styled-components example

* Immplement the initial singleton Router.

* Use the new SingletonRouter for HMR error handling.

* Use SingletonRouter inside the Link.

* Create an example app using the Router.

* Make the url parameter optional in Router.push and Router.replace

* Add a section about next/router in the README.