docs: add voice and video calling guide

[saylee21]

Closes #2329.

Adds doc/calling.md — a minimal guide for 1:1 voice and video calls
using flutter_webrtc.

Signed-off-by: Saylee Bharsakle sayleebharsakle@gmail.com

[cursor]

PR Summary

Low Risk
Documentation-only change with no runtime or API modifications.

Overview
Adds doc/calling.md, a new developer guide for 1:1 voice and video calls in Flutter apps using the Matrix SDK and flutter_webrtc.

The doc covers dependency setup, a sample WebRTCDelegate implementation (media devices, peer connection, ringtone, call UI hooks), wiring VoIP(client, delegate), placing and receiving calls via inviteToCall / handleNewCall, and a call state reference table. It points readers to the VoIP module README for group calls/LiveKit and notes automatic TURN via client.getTurnServer().

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[td-famedly]

I would suggest adding the MatrixRTC version of this not the legacy calls which are deprecated