Add guidelines for accessibility in documentation

[thibaudcolas]

Read the proposed changes as HTML. Builds upon #17338, adding guidelines specific to writing documentation. This is in the spirit of B.4.2. Ensure that documentation promotes the production of accessible content from the ATAG 2.0 standard.

[knyghty]

This looks pretty nice. I think there is value in adding this but I think this is advocating a bit too much for spreading stuff out in the docs rather than having a tightly focused a11y section.

FWIW I think there's value in both of these things but we should try to make sure anything dispersed throughout the rest of the docs are beginner-friendly and not confusing.

[Scotchester]

I agree that it's quite valuable to document accessibility considerations in the parts of the docs people are most likely to be seeing frequently – in references, alongside code examples, etc. – rather than cordoning them off into a separate accessibility page that people are unlikely to visit unless they are already interested in accessibility specifically.

I also think, to the point that @knyghty made, it would be helpful if we did make some of the example changes alongside this PR, and then we could point to them in the new docs here to give people a quick reference for how simple it can be to include accessibility considerations in the docs they are adding.

[thibaudcolas]

Now that #17338 is merged – updated this PR to simplify the proposed additions, and address all feedback so far. There is one point of feedback outstanding from @Scotchester:

it would be helpful if we did make some of the example changes alongside this PR, and then we could point to them in the new docs here to give people a quick reference for how simple it can be to include accessibility considerations in the docs they are adding.

I’m hesitant to proceed with this unless others agree this is the right think to do. Now handing over to @erosselli for review 🌈