Add ATAG statement to the about/accessibility page

[rianrietveld]

The Accessibility Team (Joe Dolson) wrote an ATAG statement, to be added to ​https://wordpress.org/about/accessibility/

This ATAG statement is sometimes requested for larger public service or government websites using a CMS. It would be great when we can point requests to the ATAG statement on theabout/accessibility page.

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ATAG Statement

The [Authoring Tool Accessibility Guidelines version 2.0](​https://www.w3.org/TR/ATAG20/), or ATAG, are a set of guidelines that govern how to create a tool for creating web pages that is both accessible and encourages the creation of accessible content.

The [WordPress accessibility coding standards](​https://make.wordpress.org/core/handbook/best-practices/coding-standards/accessibility-coding-standards/) require new or updated code to meet the standards of the Web Content Accessibility Guidelines (WCAG) version 2.0 at level AA. ATAG conformance is an additional goal beyond WCAG compliance.

An application that's fully conformant to ATAG 2.0 must not only be usable by people with disabilities, but it should enable and encourage all users to create accessible content, and assist them in repairing accessibility mistakes, without requiring any additional tools or add-ons.

WordPress is not currently conformant with ATAG 2.0, but pledges to pursue features that are accessible and which help users create accessible content in pursuit of the long-term goal of ATAG compliance.

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[rianrietveld]

Additional:
In the text

 but it should enable and encourage all users to create accessible content

the words enable & encourage could be strong
like:
but it should enable and encourage all users to create accessible content,

[obenland]

In 7388:

Main: Add ATAG statement to a11y page

Props joedolson, rianrietveld.
Fixes #3705.