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. .bd-search is ignored, it comes from Algolia (third-party).
3. .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).
4. [disabled] and [readonly] for contraqst issues, should handle that (#566)
5. .modal since it's flagged as aria-hidden containing interactive content: this is how the modal works…
6. #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 (third-party, sort of).
7. .bd-example nav is flagged too for the same reason, but it's meant to document nav usage, so…
8. .chroma is for code block using Hugo's syntax highilighter (#567)
9. [id*='tarteaucitron'] for, well, Tarteaucitron, which is a nightmare to fix at some point (third-party)

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