Cross link Interop Accessibility documentation from other relevant WPT locations to improve discoverability

[cookiecrook]

Cross link @rahimabdi's Interop Accessibility documentation:

• https://github.com/web-platform-tests/interop-accessibility/blob/main/README.md

From:

• readme files in each of the accessibility-relevant wpt test directories (accessibility, accname, etc)
• main wpt documentation source
  • https://github.com/w3c/aria/blob/main/documentation/wpt.md
  • perhaps add one or more accessibility.md files?
  • links from other .md files?