[docs] Set the right site_url

Though the entry point is docs.gotosocial.org, that's redirected by RTD
to docs.gotosocial.org/en/latest/ which is where the actual site is
served from. However, other URLs like docs.gotosocial.org/admin aren't
redirected to docs.gotosocial.org/en/latest/admin. They just 404.

Without us including the /en/latest/ all the generated og:img URLs as
well as the link rel=canonical result in URLs that all 404.

This means that currently the social cards aren't working well, but
indexing the docs site by search engines is probably also partially
broken, since our sitemap.xml is also pointing at things that don't
exist.

mkdocs.yml

@@ -1,5 +1,5 @@
 site_name: GoToSocial Documentation
-site_url: https://docs.gotosocial.org
+site_url: https://docs.gotosocial.org/en/latest/
 theme:
   name: material
   language: en