Plugin compatibility testing docs

[benlk]

Changes

• Adds file to docs to list plugin compatibility testing items, for Test compatibility with new Co-Authors Plus release #1706
  • Co-Authors Plus
  • Yoast SEO
• Removes TGM Plugin Activation docs, because TGMPA was removed from Largo
  in PR Remove list of recommended plugins, and the TGMPA plugin included in Largo. #1580 for issue Update list of recommended plugins #1570

Testing

1. workon largo or whatever python virtual environment you're using
2. grunt docs
3. In a new shell, go to docs/_build/html/ and run python -m SimpleHTTPServer. Make a note of the URL given.
4. In your browser, visit the URL. The path /developers/plugin-compat.html should show these docs.