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, security, or data-path impact.

Overview
Adds doc/calling.md, a new developer guide for voice/video in Flutter apps on top of the Matrix Dart SDK.

The doc positions MatrixRTC (GroupCallSession, matrixRTCEventStream, MeshBackend / LiveKitBackend) as the recommended path and marks legacy WebRTCDelegate / CallSession as deprecated while still showing a minimal delegate implementation for media. It walks through dependencies, creating and entering a group call, handling typed RTC events, mute/screenshare/reactions, MSC4075 incoming ring notifications, leaving a call, and includes a MatrixRTC event reference table plus links to VoIP README and TURN via client.getTurnServer().

^{Reviewed by Cursor Bugbot for commit 0cfeee1. Bugbot is set up for automated code reviews on this repo. Configure here.}

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ saylee21
❌ sayleeee1
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[td-famedly]

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

[saylee21]

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

Updated the guide to use MatrixRTC — GroupCallSession, matrixRTCEventStream, MeshBackend/LiveKitBackend, and MSC4075 notifications. Please take another look.