Major changes in this PR to how the About page is rendered.
* Bringing back the static, serverside-generated About page from v3.
This involved reverting a lot of code and modifying some of the
variables names to match changes in v4.
* Update the table of contents generator to also parse markdown
* Change a bunch of in-app routing to redirect to the static About page
instead of the React component route
* Incorporate @ClearlyClaire's [open
PR](https://github.com/mastodon/mastodon/pull/20808) for a setting that
lets admins choose to make the explore page their non-logged-in landing
page instead of About (but About is the default)
Co-authored-by: Claire <claire.github-309c@sitedethib.com>
357186376c