Add fragment links for sections

[bgins]

The site currently does not expose fragment links to sections within a page. This makes it challenging to link to a specific section.

The rendered HTML contains ids that could serve as the basis for fragment links. For example, we can link to: https://docs.everywhere.computer/homestar/configuration/#rendezvous-discovery.

The typical UI is displayed alongside the section title like:

which is displayed when hovering over the section title. On clicking on the #, the link is copied to the clipboard.