Add sitemap

- Update footer
  - Add link to sitemap
  - Remove note about Sphinx.
     It's clutter since it's mentioned in other places that we use it
- Add sitemap page
  - We are showing both document titles and section headers in the
   documents. Showing only titles kinda defeats the purpose of a sitemap.
  - Note that this makes the sitemap much larger, so users are advised to
    search through it using their browser's search (e.g. Ctrl+f).