SEO for MSCs

Nobody has claimed this yet.

In https://www.youtube.com/watch?v=A4k7DVIK5TE, @ara4n mentions "Google up [MSC3401](https://github.com/matrix-org/matrix-doc/pull/3401)", however, because google doesnt index pull requests, direct links to MSCs dont ever show up in search results.

There's a two-part solution to this, the first is to provide (generated) pages that're linked by https://spec.matrix.org/v1.1/proposals/#proposal-tracking, that explain and fill in some information from the PR.

Google can index that index page, but it will not send users to it once it finds the MSC, and even if it did, the user would be dumped inside a long list, using ctrl+f or scrolling to find the one it wanted to actually go to.

As such, the bot can probably generate these pages, and provide a link to the actual PR inside of them.

Secondly, it would be useful to provide [sitemaps](https://developers.google.com/search/docs/advanced/sitemaps/overview) next to this index, which google can scan efficiently and quickly find new MSC pages like above, which would help improve results.

Start with the proposal-tracking page at spec.matrix.org/v1.1/proposals/#proposal-tracking and inspect how MSC information is generated and linked. Review the linked MSC3401 pull request and the sitemap guidance to define the generated page and sitemap outputs; done means individual MSCs are indexable, link back to their pull requests, and are discoverable through a sitemap.

Written by the indexing model from the issue text.