Add more specific guidelines on when to include accessibility checks

What does this MR do and why?

I was experimenting with adding accessibility checks in some of the more complex feature tests. I decided to rewrite what we already had on When to add accessibility tests. It should help contributors to decide on the approach. I opted for more descriptive paragraphs, instead of a list.

cc: @ealcantara @rchanila @sdejonge @deepika.guliani @ramistry

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Relates to #420595