Move content to accessibility page

Problem/Motivation

In #3521924: Convert Coding Standards to GitLab pages, a placeholder gitlab page was created for the accessibility coding standards on d.o.

More recently, the corresponding d.o. page was changed indicate that the content has moved to the new page. However, the new page is mostly empty.

Benefits

If we adopted this change, the Drupal Project would benefit by ...

Clarity on the latest recommendations from the accessibility team.

Three supporters required

1. https://www.drupal.org/u/{userid} (yyyy-mm-dd they added support)
2. https://www.drupal.org/u/{userid} (yyyy-mm-dd they added support)
3. https://www.drupal.org/u/{userid} (yyyy-mm-dd they added support)

Proposed changes

Create an MR with all proposed changes.

2. Repeat the above for each page or sub-page that needs to be changed.

Remaining tasks

1. Create this issue in the Coding Standards queue, using the defined template
2. Add supporters
3. Create a Change Record
4. Review by the Coding Standards Committee
5. Coding Standards Committee takes action as required
6. Discussed at a Core Committee meeting, if it impacts Drupal Core
7. Final review by Coding Standards Committee
8. Documentation updates
   1. Commit the MR
   2. Publish change record
   3. Remove 'Needs documentation edits' tag
9. If applicable, create follow-up issues for PHPCS rules/sniffs changes

For a full explanation of these steps see the Coding Standards project page