Update the Known Issues with the new Pantheon Compatibility feature

[scottbuscemi]

Expected behavior

• Documentation notes that the Pantheon mu-plugin now adds a compatibility layer that applies many of these fixes
• Specific fixes that are auto-applied are noted in the doc

Actual behavior

• Documentation does not mention the mu-plugin

Suggested Resolution

• Update the docs
• [process] When new issues are reported, and this page is updated, open an issue in the mu-plugin repository to apply the fix (or the warning/error) to the mu-plugin's compatibility layer (see Add WP Cerber to compatibility layer pantheon-mu-plugin#52 and Add PhastPress to compatibility layer pantheon-mu-plugin#53)

[jazzsequence]

@scottbuscemi We've gotten two new plugins to add to the list (hard no in one case -- the other one might be auto-fixable if we can apply settings), so I took a pass at fleshing this docs ticket out. If there's anything else you think we should put in the docs page on this, let me know. I think we should hit it sooner rather than later considering there are new things to add.

[rachelwhitton]

Breaking out automation for the new process into a separate issue is a good idea, but we can update codeowners for this doc page in the meantime to list @jazzsequence and @scottbuscemi and/or a PDE squad @pantheon-systems/site-experience