replace nagios with prometheus in our documentation

during the nagios retirement, i came across the "power grep" and "update docs" tasks which are normally relatively easy to do (but certainly can be tricky)...

but for nagios, this is a whole other ball game: we refer to "nagios" (or icinga?) everywhere. pretty much all service pages have pager playbooks that refer to nagios checks, and we have docs about it everywhere.

let's go through our docs and clean this up.

note that this is different from the #41655, which only concerns prometheus documentation, not rewriting the nagios stuff for prometheus.

task list:

• grep for nagios and icinga over the wiki and replace easy marks (not playbooks)
• mark all playbooks refering to nagios/icinga as deprecated, we have an issue to specifically have playbooks everywhere, see prometheus-alerts#16
• power grep over all source code
• look around gitlab for nagios-related projects and archive (tor-nagios already archived)
• mark tor-nagios.git as "deleted" in mrconfig