Add linkage between attributes and HTML-AAM

[domenic]

For elements, thanks to @carmacleod's work in #5077 we have linkages from HTML's element definitions to the corresponding parts of HTML-ARIA and HTML-AAM.

The discussion in #7796 revealed that we have no such setup from attributes, and that is causing some confusion. In particular, it's unclear who should be defining the accessibility mapping for attributes like inert="".

Our conclusion on the HTML triage call (#7919) was that HTML-AAM should define the accessibility mapping for attributes, just like it does for elements. (In fact, it already does, although not yet for inert="".) And that, just like for elements, HTML should add cross-links.

In this issue we can discuss what form such cross-links would take. It's less obvious for attributes than for elements, since there's no clear "Attribute definition" block we can slot in such links... Also, I didn't realize it during the meeting, but there are a ton of attributes with accessibility mappings. Not just a few big ones like inert="", but also abbr="", accesskey="", alt="", and autocomplete="" just in the As. So it could be difficult to slot in all these links.

Possible locations we could use:

• The attribute index
• In the element definition (example), maybe something like "abbr — Alternative label to use for the header cell when referencing the cell in other contexts (accessibility mappings)"
  • But then what about the global attributes?