Doc: Big Picture Overview, Forced Logs, Operations

[oxzi]

doc/01-About.md: Big Picture

Inspired by the "Big Picture" section in the Icinga Notifications
documentation[^0], such a section was introduced for Icinga DB as well.
It aims to describe the essential components in an Icinga DB setup along
the architecture diagram for novice users. This hopefully helps new
users to understand what Icinga DB actually is and how it interconnects.

[^0]: https://icinga.com/docs/icinga-notifications/latest/#big-picture

---

doc/03-Configuration.md: Mention forced log events

The logging was slightly modified in #920, resulting in a few log
messages being shown regardless of the user configured log level. Since
this may not be the expected behavior, it is now mentioned in the docs.

---

doc/07-Operations.md: New Operations section

The newly introduced Operations section is a first attempt at an
operational Icinga DB documentation.

It covers
- essential Icinga DB monitoring,
- backups and corner cases for MySQL/MariaDB,
- optional systemd service restarts after failures,
- MySQL/MariaDB configuration options, including AWS RDS and Galera,
- and memory overcommitment for Redis.

As such a section can never be completed, this documentation is a target
for continuous improvement.

Fixes #745.

---

Fixes #745.

[theFeu]

I added some comments with suggestions for minor improvements :)
This reads well and is very nicely done. Thank you!

[theFeu]

👍