add documentation about the "room" feature

docs/features/rooms.md

+# Rooms
+
+Rooms are used to link another entity to a specifc location.
+
+Rooms can represent different types of physical and virtual locations, e.g.:
+
+- a workshop room or
+- a lecture hall
+- or a Big Blue Button meeting.
+
+## Attributes
+
+A room has different attributes - those are the most important ones:
+
+| Attribute | Values                                                                                                                                                                    | Writeable by                         | Description                                                                                                                   |
+| --------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------ | ----------------------------------------------------------------------------------------------------------------------------- |
+| `public`  | `true` / `false`                                                                                                                                                          | Admins via `/c3admin`                | Public rooms are displayed in the curated calendar schedule, non-public rooms are displayed in the assembly calendar schedule |
+| `blocked` | `true` / `false`                                                                                                                                                          | Admins via `/c3admin`                | Blocked rooms are hidden publicly.                                                                                            |
+| `type`    | One of: `lecture hall`, `stage`, `workshop room`, `other`. There are further types that are no longer in use: `online`, `outside`, `project`, `big blue button`, `hangar` | Chosen on creation in the backoffice | Allows to specify what kind of a room it is.                                                                                  |
+
+## Room shares
+
+Rooms default to be limited to the assembly they are created for. However additional "room shares" allow for sharing rooms:
+
+- with selected other assemblies,
+- all assemblies or
+- self organized sessions
+
+A new room share can only be created via `/c3admin` currently. A new "Room Share" entry will alow to specify:
+
+- which room should be shared,
+- if the share is enabled,
+- whom the room should be shared with and
+- an optional timeframe to limit the share to (format `YYYY-MM-DD HH:MM:SS`).
+
+## Room reservation
+
+Rooms are automatically reserved/occupied once an event is linked to the room. There can not be any overlapping events in a room.