Website: Fix HTML validation errors

Ids like documentation, download, support, about, community in the
top menu collide with the same ids inside documents, breaking
validation and <label for=...>. Match footer.html and prefix them
with the section name (e.g. header-community-title).

Note: id=""name"" is left as is, so name anchors are implicitly
unavailable on the site.

Reviewed by: carlavilla
Approved by: carlavilla
Differential Revision: https://reviews.freebsd.org/D59857
PR: 297509