Add note to clarify the workspace join link functionality

[mountiny]

### Problem

Coming from here, we have agreed that on the workspace share page, we do not provide enough details for the admin about what the link/QR code does.

The link will allow users to join the workspace upon visiting while being signed in. A message will appear in the admins room for each join request to confirm the addition (unless its a domain controlled account in which case we pre-approve the requests but I think we can omit that information).

Solution

Lets add some message to the share page that will make this clear, maybe with some icon/ illustration too