docs: Reorganize and revise moderated sessions (gravitational#32420)

* Get revision for moderated sessions into the 15.x directory

* Add backticks and < > around mode

* Add keywords

* Fix typo

* remove extraneous line

* Update connect your client with changes related to moderated sessions

* updates from Zac's review

* fix typo

* fix typo

* You can only require session moderation using Enterprise editions

* Clarify the Teleport Enterprise requirement

* Add paragraph about role mapping and moderated sessions

* updates from Paul's review

* updates from Paul's review-2

* Replace caveat about join_sessions/deny rules