Website: Fix HTML validation errors

Remove the <a> wrapper around each <label> in the top-level menu,
which triggered HTML validation error on every documentation page.

Extend the .menu-item a rules to .menu-item-description so the label
keeps the same appearance and the sub-menu hover trigger still works.

Mirrors the website fix in a8654f311.

Reviewed by: carlavilla
Approved by: carlavilla
Differential Revision: https://reviews.freebsd.org/D60039
PR: 297509