Add Perfect Negotiation sample page

[henbos]

In M87, Chromium-based browsers support the Perfect Negotiation pattern - for more info, see PSA.

We should add a web page that illustrates this pattern, similar to this jsfiddle (referenced by the PSA).

[henbos]

@jan-ivar @alvestrand I think we can improve on the demo and add it to the samples page, but I don't know when there would be time to do that (for me personally)

[henbos]

I think we should make the sample page such that the Perfect Negotiation logic is a separate .js file like a "library" that you make use of in the sample page, to show how "plug-and-play" the pattern is.

[SCG82]

Perhaps someone can add tests and polish my draft PR #1384 (unfortunately I won't have any time in the near future to work on it).

[henbos]

Thank you @SCG82, it's now at https://webrtc.github.io/samples/src/content/peerconnection/perfect-negotiation/