GroupCall/Conference example

Hello, i'm now implementing matrix support in my app by this matrix-js-sdk.
Chat & 1v1 video calls works fine.

But i can't found any GroupCall/Conference example.

This not work:
```

let placeCallFunc = x => {
if(x === null) return false;

x.updateParticipants()
x.initLocalCallFeed();
x.placeOutgoingCalls();
return true
}

let c = this.matrixClient.getGroupCallForRoom(roomId)
placeCallFunc(c) || this.matrixClient.createGroupCall(
roomId,
devices["videoinput"] ? "m.video" : "m.voice",
false,
"m.room",
true,
{
maxPacketLifeTime: null,
maxRetransmits: null,
ordered: true,
protocol: "udp"
}).
then(
c => {
placeCallFunc(c)
```

Can your place correct order for using methods for create group video cals?
Or any working example.

...
I research code of element, they use jitsi for conference call, i want make it by pure matrix-js-sdk.
This try not work too:
```
if(groupCall === null && roomId) {
client.createGroupCall(
roomId,
"m.video",
false,
"m.prompt");

groupCall = client.getGroupCallForRoom(roomId)
await sleep(10);
}
if(groupCall === null) return
//groupCall.addListener("feeds_changed", onFeedsChanged);
groupCall.enter();
addListeners(groupCall);
```

Test part of matrix-js-sdk use simple create group call and enter, and commented that sleep 10 ms to "wait for calls" but i have not any call.

Start with the matrix-js-sdk group-call entry points named in the issue: createGroupCall, getGroupCallForRoom, enter, and the participant and feed methods. Compare their order with the existing group-call tests, which use create and enter, and check the related Element/Jitsi context. Done means the supported pure matrix-js-sdk sequence is clear and demonstrates a working group video call.

Written by the indexing model from the issue text.