Add group-joined and group-left to the websocket tutorial

[Protected]

These events are issued when the user connected to the websocket effectively becomes a member of a group or effectively stops being a member of a group.

The payload is always: {"groupId": "string"} .

[Foorack]

Thanks for this Issue!

Due to very technical reasons, this is a "Tutorial" issue, and not part of the OpenAPI specification, so will move this to the front-end repository.

[VinyarionHyarmendacil]

I added these in #140, so I think this could be considered as having been addressed.

[Foorack]

@VinyarionHyarmendacil Does the content-refresh event really cover joining or leaving a group? They should be different events, therefore not solved by #140.

[VinyarionHyarmendacil]

@Foorack whoops, meant #134