review prometheus documentation after service overhaul

We're moving a lot of things around in Prometheus. Make sure the Prometheus documentation is up to date in the wiki, in particular, perform the following checks:

priority A

Those need to be done as part of %TPA-RFC-33-A: emergency Icinga retirement, before we give out training (#41767 (closed)).

• overall document structure review (done until pager playbooks section)
• quickly, review monitoring and testing section, to see if there's any urgent changes to be made there
• how to scrape a new target? (present, but messy)
• how to add an existing alert to prometheus
• document IRC channel
• how to write an alert?
• "where is my nagios check?" howto
• document silences
• how to create a blackbox check?
• document blackbox exporter oddities

See also the questions raised in the training, in #41767 (closed).

priority B

those may be done after icinga is retired, as part of priority B (%TPA-RFC-33-B: Prometheus server merge, more exporters).

• sync with template.md
• review backups
• review monitoring and testing, yes, again
• review architecture
• review design, possibly copying a lot of TPA-RFC-33 in here
• storage
• queues
• authentication
• implementation
• related services
• Security and risk assessment
• Technical debt and next steps
• document TPA-RFC-33 in the wiki page (proposed solution?)
• remaining TODO items

/cc @lelutin