Docs accessibility issues

On v5-dev, pa11y-ci is configured to ignore a few things.

Here's a comprehensive list of ignored rules and elements, to discuss:

1. The heading-order rule is ignored, since Bootstrap doesn't want to improve their heading hierarchy (and this is probably not a big deal, only a few level skipped).
2. 3^rdparty: .bd-search is ignored, it comes from Algolia.
3. 3^rdparty:[id*='tarteaucitron'] for, well, Tarteaucitron, which is a nightmare to fix at some point.
4. 3^rdparty:#TableOfContents is flagged as redundant nav landmark, which is generated by Hugo (so we can't change it) and moreover is not an issue at all.
5. Orange: .text-primary, .navbar-light .navbar-brand, .active, [aria-current], .accordion-button:not(.collapsed) are ignored since their contrasts aren't sufficient (#f16e00 on #fff, brand side).
6. [disabled] label and [disable + label] for contrast issues (brand side), however WCAG 2.1 does not require sufficient contrast for inactive elements.
7. .modal since it's flagged as aria-hidden containing interactive content: this is how the modal works…
8. .bd-example nav is flagged too for the same reason, but it's meant to document nav usage, so…

To maybe fine-tune our criterias, pa11y's wiki has a comprehensive list of ignorable rules.