Fix HTML validation errors in docs - on main page

Card titles on the documentation homepage were rendered as <h5>
right after the <h2> from global-search.html, skipping two
heading levels. The page also had no <h1>.

Add a visually hidden <h1>{{ .Title }}</h1> before the search
partial, render card titles as <h3 class=""card-title"">, and
rename the .card a h5 selector to .card a .card-title with
padding-top and padding-bottom set, so the look stays
the same. Add a .visually-hidden helper to global.scss.

Differential Revision: https://reviews.freebsd.org/D60069
PR: 297509