Concept docs: reducers

[gefjon]

How to define a reducer. Link to tables page from #3843 section about how to query and mutate tables from within reducers. Note about reducers running isolated, without the ability to affect the outside world, link to procedures page. Sections (or sub-pages?) for the special / lifecycle reducers client_connected, client_disconnected, init. Link to tables page section on scheduling. Warning that reducer arguments are broadcast to clients. Section on how to invoke reducers from clients and how to register callbacks.