docs: Introduce Kubernetes/FlowFuse observability documentation

[ppawlowski]

Description

This pull request introduces the observability documentation.

Related Issue(s)

Closes https://github.com/FlowFuse/engineering/issues/114

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production
• Link to Changelog Entry PR, or note why one is not needed.

Labels

• Includes a DB migration? -> add the area:migration label

[knolleary]

Good start for sure.

The only point we get to FlowFuse specific information is linking off to /docs/admin/monitoring.md

I think it would be helpful to have a bit more on that page on how to add the stats endpoint to prometheus. Right now that is left entirely to the reader to figure out. I don't think it needs to be lots, just enough to get it done.