Add documentation for xymon

[gctucker]

The xymon service is used to monitor KernelCI services. Some documentation should be added to https://kernelci.org/docs/admin/ so sysadmins know how it's set up and how to keep it working.

[gctucker]

@montjoie As we're now using Zabbix for monitoring, this can be closed. Please also make sure you remove all the xymon setup wherever it was installed.

[montjoie]

All xymon stuff is removed from staging.

[gctucker]

@montjoie Where was it by the way?

[montjoie]

It was only deployed on staging. I will check prod soon, but in my memory, I never started deploying on it due to missing answer to "how to do a secure link beween them".

[gctucker]

But I mean, where on the staging server was it deployed? Under which user, and how was it deployed?

[montjoie]

It was deployed by hand via:

• the xymon-client apt package (which is now removed via apt --purge)
• a docker with all data/dockerfile under ~/xymon for kernelci user. (the whole directory is now deleted)
  I did a find on staging to verify nothing remains