Fix HTML validation errors in docs site header - menu ids

Same fix as 10633eb for the website header, applied to the
documentation header.

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

Differential Revision: https://reviews.freebsd.org/D60059
PR: 297509