Documentation pages emit "The element label must not appear as a
descendant of the a element" for every top-level menu item, because
each label is wrapped in <a href="#">.

Apply the same fix already merged for the website in a8654f311
("Website: Fix HTML validation errors"):

• drop the wrapping anchor and
• extend the existing .menu-item a rules to .menu-item-description,

so the label keeps the same appearance and the sub-menu hover still works.

• header.css: add .menu-item-description to the rules that previously targeted .menu-item a only (padding/color, text-decoration, font-weight, :hover, and the +.sub-menu hover trigger), in both the desktop and max-width: 1100px blocks.
• site-header.html: remove the <a href="#" aria-label="..."> wrapper around each <label class="menu-item-description">.

No visual changes expected — mirrors the website fix.