Docs: Explain main content node detection in visual diff (#12586)
Adds a new section to the visual diff documentation explaining how Read
the Docs detects the main content area of HTML pages for comparison.
## Changes
- Added "How main content is detected" section explaining the 4-step
priority order
- Reorganized limitations section to separate content detection
explanation
- Provided recommendations for improving detection in non-standard
documentation
This helps users understand why certain changes may or may not appear in
the visual diff and what they can do to improve detection accuracy.
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Co-authored-by: Manuel Kaufmann <humitos@gmail.com>