Documentation: Distributed Monitoring misses Icinga DB, DB IDO only

Is your feature request related to a problem? Please describe.

The Distributed Monitoring documentation does not mention the Icinga DB, but only refers to HA with DB IDO which, however, seems to be deprecated.

There is some HA documentation in the Icinga DB's own Distributed Setup documentation, but it is kind of short and does not neatly fits the main Icinga 2 documentation.

Describe the solution you'd like

The documentation should be updated to also cover the Icinga DB and mark the DB IDO parts as outdated or even start to drop them.

Describe alternatives you've considered

N/A

Additional context

N/A

Start by comparing the Distributed Monitoring documentation with the linked Icinga DB Distributed Setup documentation, especially the HA sections. Determine how Icinga DB should be covered and which DB IDO material is outdated; the documentation is done when the main guide covers Icinga DB and clearly marks or removes obsolete DB IDO guidance.