Populate "WebRTC Browser-to-Browser" in transport/webrtc

[p-shahi]

Context

The libp2p maintainers have continued specifying the WebRTC specification and are adding support for browser-to-browser connectivity:

• The Working Draft version of the specification is in progress: feat(webrtc): add WebRTC (prev. browser-to-browser) spec specs#497.
• Per the working draft, three libp2p implementations are estimated to add support during Q2 2023 (go-libp2p, js-libp2p, and rust-libp2p).
• WebRTC interoperability testing across implementations is also started mid Q4.

Done Criteria

• Populate the "Browser-to-Browser" section in the WebRTC transport document.
• Audit and edit the original WebRTC transport document to ensure the general content does not revolve browser-to-server.
• Add a short comparison sub-section comparing the differences in connectivity

Why Important

• Expanding the ways libp2p enables connectivity (into browser connectivity) is a primary goal of the libp2p project, WebRTC is centre to that. As we finish specification and implementation, we need corresponding documentation for the transport and it's use-cases.
• This continues the transport documentation for WebRTC started by Document "WebRTC: Browser to Server" #211.

[salmad3]

Marking as blocked until #264 is pushed.