Documentation pages emit validation errors about skipped heading
levels in the footer: <h1> is followed directly by <h3> for each
column title, which the validator flags on every page.
Apply the same fix already merged for the website in c33d966fc2
("Website: Fix HTML validation errors"):
- site-footer.html: replace <h3 class="column-title"> with <p class="column-title" id="..."> for the about/documentation/ community/legal columns, and add aria-labelledby on the matching <section> so the titles remain accessible names.
- footer styles: add margin-bottom, font-weight, line-height, padding-top and padding-bottom to .column-title, restoring what <h3> previously provided via browser defaults.
No visual changes expected — mirrors the website fix.