Review and update Accessibility docs

[sarina]

• Reviewed and updated Accessibility Guidelines and Accessibility Best Practice Checklist - both passed
• Reviewed Design for Mobile - got information from mobile WG in slack.
• Removed Accessibility Checker - it is a paid subscription and requires private code on the edx.org side. Reference/conversation: These plugins require a paid TinyMCE subscription frontend-components-tinymce-advanced-plugins#27
• Reviewed/updated Manage video transcripts - passed

[sarina]

Note: Update Mobile supported blocks per the mobile team

Yes, this table is 90% accurate.
I just checked the code, and we also support:
SURVEY (idk what is this, but it's added to supported blocks for both platforms)
OPENASSESSMENT

• We also support content xBlocks:
  html
  video
  discussion