Fix HTML validation errors in doc site footer

Replace the <h3> column titles with <p class=""column-title""> to stop
validation errors about skipped heading levels on every page, and add
aria-labelledby on each section so the titles are still exposed as
accessible names.

Restore the styling h3 previously received from the browser
(font-weight, line-height, padding) on .column-title so the footer
looks unchanged.

Mirrors the website fix in c33d966fc2.

Reviewed by: carlavilla
Approved by: carlavilla
Differential Revision: https://reviews.freebsd.org/D60040
PR: 297509