Migrate docs to Docusaurus v3 (#6133)

**What's the problem this PR addresses?**
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The docs website is on Docsaurus v2 which is not the latest major
version

Closes #5918
Closes #6153

**How did you fix it?**
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Migrate to Docsaurus v3.

There are other improvements to the build process and website I'd like
to make, but I have left them out and kept most of the logic intact
since the migration is quite a large undertaking already.

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Author: clemyan

.gitattributes

@@ -16,7 +16,7 @@ packages/yarnpkg-libzip/sources/libzipSync.js           binary linguist-generate
 
 # Hide .yarn and docs from GitHub's language detection
 /.yarn/**               linguist-vendored
-/docs/**                linguist-documentation
+/packages/docusaurus/** linguist-documentation
 
 # Mark built javascript binaries as generated
 # Without this, the repo is marked as >90% javascript, while most code is actually typescript

.gitignore

@@ -38,9 +38,6 @@ package.tgz
 /packages/yarnpkg-libui/sources/**/*.js
 /packages/yarnpkg-libui/sources/**/*.d.ts
 
-# Only contains the Webpack cache
-/packages/docusaurus/.yarn
-
 # Used by /scripts/stable-versions-store.js
 /scripts/stable-versions-store.json