Predefined resources

Greetings to all, first of all congratulations to those involved in the project... I think it would be interesting to add a feature where the user enters the space with visibility of all the rooms that are public in that space, thus saving time when searching and imagining the available names.

Another interesting feature would be to insert the newly created user into a pre-defined room, such as a "Greetings" room.
Forgive me if this feature already exists, but I didn't find anything related in the documentation.

The issue names no files, tests, or entry points. Start by checking the Synapse documentation for existing public-room discovery and predefined-room behavior, then clarify whether both requested features are in scope. Done would require an agreed design and acceptance criteria for showing public rooms and placing new users in a predefined room.

Written by the indexing model from the issue text.