Documentation updates for >= 5.4

[jeremydmiller]

Got a little behind on things.

• [x] IChangeListener for the async daemon
• [x] Recommendation of computed index over duplicated field
• [x] Executing custom SQL in DocumentSession
• [x] IgnoreIndex()
• [x] Easier mechanism to add custom database objects through Marten
• [x] DeleteObjects(IEnumerable<object> documents)
• [ ] Maybe a bit more on tombstone events?
• [ ] New base classes for aggregated projections
• [ ] Version convention on aggregations
• [ ] CLI with WebApplication bootstrapping
• [ ] Event store FetchForWriting()
• [ ] New IEventStream mechanics
• [ ] Mention DeadLetterEvent in the event store storage docs