TINYDOC-3645: Update the accessibility conformance page with the latest VPATs and review feedback.

[kemister85]

Ticket: TINYDOC-3645, TINYDOC-3650

Site: Staging branch

Changes:

• accessibility-conformance.adoc: list the latest VPAT for each major version (TinyMCE 8 and TinyMCE 6) with per-version WCAG coverage, and remove the Published column and the Known limitations section.
• Apply the TINYDOC-3650 review feedback: remove the Overview list, add VoiceOver, and reword the authored content note, the screen reader paragraph, and the support contact.
• attachments/vpat/: replace the TinyMCE 8 report with the 8.9 report (same filename) and add the TinyMCE 6.7 report.

Pre-checks:

• Branch is correctly prefixed:
• modules/ROOT/nav.adoc has been updated (if applicable).
• Files have been included where required (if applicable).
• Files added for New product features include a release note entry.
• Build passes without console errors, warnings, or issues.

Review:

• Documentation Team Lead has reviewed.

[metricjs]

various nitpicks but overall lgtm