docs: relay flow documentation

Expanded introduction to explain relayMessage and its abstraction of window.postMessage.
Detailed forwarding of messages to background service worker and processing by message flow handlers.
Introduced sendToBackgroundViaRelay for sending messages and awaiting responses, with unique instance ID handling.
Added GitHub link for viewing function implementations.
Highlighted as an alternative to the "externally_connectable" Chrome extension method.
Provided detailed steps for setting up a relay with relayMessage.
Completed instructions on sending messages using sendToBackgroundViaRelay.

PlasmoHQ / docs