Add WCAG 2.2 to accessibility specification

[mattgarrish]

This pull request makes a few editorial changes to keep pace with the latest WCAG recommendation:

• it adds examples of the three possible 2.2 conformance strings
• it updates the conformance strings in the examples to reference 2.2
• it updates a link to the 2.1 techniques document to 2.2 (there is no auto-updating link for this document)

Fixes #2629

• preview
• diff

[iherman]

I expect these changes do not affect conformance, ie, this is a class-2 change, right? Better set the flag...

[mattgarrish]

Do we also need to flag the pull requests? The issue is marked as class2.

(There aren't any other labels that would need to be applied? I don't want to screw up our errata generator.)

[iherman]

Do we also need to flag the pull requests? The issue is marked as class2.

I missed that, sorry. But I think it does not harm if the PR is also flagged. I have just done it.

(There aren't any other labels that would need to be applied? I don't want to screw up our errata generator.)

No, it looks o.k. to me...