feat: add RGAA tags to rules

[straker]

• Update tags
• Update docs
• Add tag validation

Closes: #4850

• Make new tags available for DQU documentation (after merging)

[sierra88-en]

hazaña: agregar etiquetas RGAA a las reglas

[dbjorge]

I note that this is omitting tagging from the rules that are currently best-practice that we think are RGAA-relevant (focus-order-semantics, region, skip-link, table-dupe-name). Have you already discussed/thought about the plan for those?

They're interesting since it's the first time we're handling an accessibility standard that is more strict than any WCAG standard for some rules. If we say "we only mark stuff best-practice if there's no accessibility standard that covers it, so we remove best-practice from those 4 rules as we add the RGAA tags", it'll mean that any axe-core user that's currently using something like runOnly: ['wcag2a', 'wcag2aa', 'wcag21a', 'wcag21aa', 'best-practice'] will end up with a breaking change where they start missing those rules. I don't think we should make a breaking change like that.

But if we instead say "we mark both best-practice AND the RGAA tags for such rules", there's going to be a lot of code in our own products that'll need updating for cases where we have product UI that displays/filters best-practice issues separately by checking for the presence of the best-practice tag. All those cases would need to start understanding to treat rules in this state as best-practice if-and-only-if they aren't matched by the active accessibility standard.

I think the second approach is probably what we'll have to do for backcompat's sake, but I think before we put rules into that state, we'll want to audit for products that are going to be affected and make sure we account for the extra time it'll take to make sure they update that logic as part of consuming the corresponding axe-core update.

Based on that, I agree with what you've done here in omitting those cases from the tagging in this initial PR, but we should make sure we have a tracking issue set up to track the extra work to set up those 4 additional rules.

[straker]

Here's the tracking ticket #4873