Clarify that our Debian installation docs also apply to the Raspberry Pi OS

[Al2Klimov]

• CC https://github.com/Icinga/packages.icinga.com/pull/46

[julianbrost]

Note that the Icinga DB installation instructions are given for fewer operating systems than for example Icinga 2, compare the following two:

• https://github.com/Icinga/icingadb/tree/main/doc/02-Installation.md.d
• https://github.com/Icinga/icinga2/tree/master/doc/02-installation.md.d

You'll note that Fedora is also missing for example. This is because on https://icinga.com/subscriptions/support-matrix/, we make a difference between support for agents vs. for a full master setup. So I'm not sure if makes much sense to add instructions for Raspberry Pi OS in that context.

[lippserd]

Icinga Web also lists other instructions, but ultimately all projects should include the instructions from here:

https://github.com/Icinga/package-installation-docs/tree/main

This is different from the support matrix because that is only for support, not for operating systems for which we have packages.