Re-merge Switch web-render option default to auto. Add documentation

[nturgut]

Description

Remerge the PR: https://github.com/flutter/engine/pull/23090/files Also add rendering backend related documentation to felt.

Note: we should merge framework PR first: flutter/flutter#72628

Related Issues

flutter/flutter#72377

Tests

These are config changes.

Checklist

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Reviewer Checklist

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Breaking Change

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[flutter-dashboard]

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