[Screen reader] Describe automatic properties and add FullDescription

[citelao]

Description

Today, the Screen reader guidance does not include FullDescription documentation, and it also implies that many properties must be set manually. This change fills those gaps and clarifies some aspects of designing for screen readers.

This is intended to be the first of a few edits to the accessibility section.

Motivation and Context

There are new accessibility pages in the gallery, each with concrete examples. This change clarifies some examples and adds more:

• Accessible name
  • Add a more robust description of accessible name
  • Add examples of automatic accessible name
  • Moved redundant controls to Raw view
• FullDescription, HelpText, and PosInSet
  • Add overall description of these common properties
  • Add example of FullDescription and HelpText
  • Add example of automatic PosInSet/SizeOfSet
• Visual tree
  • Add description of and motivations for visual trees.
• General
  • Add accessible headings to the headings in all examples
  • Add accessible headings to page titles

How Has This Been Tested?

Deployed locally.

• Narrator correctly narrates new content.
• Links work.

Screenshots

Types of changes

• New feature (non-breaking change which adds functionality)

[citelao]

Fixes #1227

[karkarl]

Looks good! Feel free to merge in whenever you're ready.