Doc: Admin/Usage Guide

[oxzi]

This is mostly a loose idea I had while reading various topics, especially those in the community forum. The current documentation roughly describes Icinga DB, tells you how to install and configure it, but not really how to run it on a day-to-day basis.

For example, there's nothing about backups, and I haven't found anything in the Icinga 2 docs either. Next to a cron job running mysqldump, there may be some stumbling blocks with larger databases as shown in #742 where the mysqldump LOCK over a bigger database made Icinga DB crash.

These and other potential issues could be documented for others to find, but I don't think it will scale to put them between the lines of the configuration chapter.

Thus, I would like to start a more end user/admin focused documentation section about your daily life with Icinga DB.