Icinga DB section

• IDO is mentioned once as being deprecated. I would simply drop its existence.

• This section mentions a Redis where data can be read from. But it is not stated where the data comes from, Icinga 2's IcingaDB feature.

• Since the latest Icinga DB release we have a "Big Picture" section: https://icinga.com/docs/icinga-db/latest/doc/01-About/#big-picture. We should think about if we want to move and refer to this section into this, or if we want to combine them somehow?