[policy, no patch] Update Drupal Core's Accessibility Gate

As part of reviewing the existing accessibility documentation and getting rid of some of the old Drupal 7 documentation I realized I need to address the

Goals

• updating the target from Web Content Accessibility Guidelines (WCAG) 2.1 to WCAG 2.2 (now the standard for EN 301 549)
• reduced page length
• Move to make it more consistent with the outcome-based checks of the other core gates
• Separating out ATAG
• Links to a detailed review process rather than duplicating a testing matrix
• Defines a pass, follow-up, and reasons to postpone

Using AI, I came up with these suggestions. I then brought it to Slack for discussion the #accessibity channel:
https://docs.google.com/document/d/1FnDcWGwq7cU_pNwR0sX6b24eaniboobE-lO3...

@quietone noted that for each one there should be a section for 'When is it needed'.,'Details', and 'Resources'. So I'v quickly created (needs more review still):
https://docs.google.com/document/d/1FnDcWGwq7cU_pNwR0sX6b24eaniboobE-lO3...

This can be edited in the GoogleDoc or here. I'm fine either way.