Implement section anchors in non-man pages

[waldyrious]

This is a follow-up to #1166, which was implemented in #888. It would be nice if section headers in non-man pages (e.g. https://git-scm.com/community) were linkable.

Given the ongoing work on v2 of the website, it probably makes sense to integrate these anchors in the new design, as @peff commented in #1166.

[pedrorijo91]

cc @jasonlong

[jasonlong]

Added to the project board backlog.

[phil-blain]

This would also be really helpful for the book! (the anchors exist, but are not easily discoverable by hovering as in the manpages).

[hoijui]

Ahh... indeed they exist! strangely though, they are not constructed in the same way they are constructed in Markdown by default.

In Markdown, the header with text "The Git Protocol" would become "#the-git-protocol",
but here we get "#_the_git_protocol".