Adopting explainers

[eladalon1983]

I've produced the following explainers for screen-share controls that we introduced over the last year:

• displaySurface explainer
• selfBrowserSurface explainer
• suppressLocalAudioPlayback explainer
• surfaceSwitching explainer
• systemAudio explainer

There's also one for Conditional Focus:

• Conditional Focus explainer

Shall we copy them to this repo? (I'll keep copies in the old repos to accommodate people following the old links.)

[eladalon1983]

(We can massage and extend them to remove the focus on Chrome, either making them more generic, or covering various implementations, as applicable.)

[eladalon1983]

@dontcallmedom

[alvestrand]

Elad will produce PRs, starting with conditional focus. We'll review in detail in the PR.

The screenshare "global" explainer should have links to the others.