Ability to turn off read receipts on a room-wide basis

(Came up during FOSDEM 2022)

#### Suggestion
The ability to turn off read receipts on a room-basis, this'd be both a hint to servers and clients to not process nor display read receipts for a particular room.

Start by reviewing the Matrix specification's existing treatment of read receipts and room-level behavior. Define the protocol semantics for disabling processing and display in a room, including how the server and clients should interpret the setting; the work is done when the specification clearly documents the behavior.

Written by the indexing model from the issue text.