Documentation: Monitoring Basics: Add/Fix livestatus and add DB IDO incl health checks.

Fixes #6356
Fixes #6358
Fix help text for icinga2-{en,dis}able-feature.

Refs #6315
Documentation: Fix default enabled features, add troubleshooting hints.

Fixes #6357
Documentation: Fix Icinga Wiki url.
Documentation: Fix typo.

Refs #6357