Support initiating Jitsis via self-hosted jitsis

https://github.com/vector-im/element-web/blob/develop/docs/jitsi.md says that iOS doesn't support instantiating jitsis on custom self-hosted jitsi deiployments. https://github.com/vector-im/element-ios/issues/3150 implies we support joining them, however.

Did we ever hook up instantiating them?

Start with docs/jitsi.md and element-ios issue #3150, then trace the existing iOS path for joining Jitsi deployments and any separate initiation path. Done means confirming or adding support for initiating meetings on custom self-hosted Jitsi deployments, with behavior aligned to the documented support.

Written by the indexing model from the issue text.