feat(v2): docs versioning ❄️🔥 (#1983)

* wip: versioning

* wip again

* nits lint

* refactor metadata code so that we can have inobject properties optimization, fix typing

* remove buggy permalink code

* modify versioned docs fixture such that foo/baz only exists in v1.0.0

* refactor metadata.ts so that there is less transformon object

* more refactoring

* reduce test fixtures, refactoring

* refactoring readability

* finish metadata part

* refactor with readdir

* first pass of implementation

* fix mdx laoder

* split generated routes by version for performance & smaller bundle

* test data for demo

* refactor with set

* more tests

* typo

* fix typo

* better temporary ui

* stronger typing & docsVersion command

* add 100% test coverage for docsVersion command

* more test and delete manual docs cut

* cut 2.0.0-alpha.35 docs

* cut alpha.36 instead

* copyright

* delete versioned docs

* stronger test on metadata

* update typo

website/docs/advanced-plugins.md

@@ -133,7 +133,7 @@ module.exports = {
         /**
          * URL for editing website repo, example: 'https://github.com/facebook/docusaurus/edit/master/website/'
          */
-        editUrl: 'https://github.com/repo/project/website/',
+        editUrl: 'https://github.com/facebook/docusaurus/edit/master/website/',
         /**
          * URL route for the blog section of your site
          * do not include trailing slash

website/docs/markdown-features.mdx

@@ -67,7 +67,7 @@ The headers are well-spaced so that the hierarchy is clear.
 
 This will render in the browser as follows:
 
-import BrowserWindow from '../src/components/BrowserWindow';
+import BrowserWindow from '@site/src/components/BrowserWindow';
 
 <BrowserWindow url="http://localhost:3000">
 <h2>Hello from Docusaurus</h2>