docs(ci-runners): add ssh steps and info about shards

Rehabrequested to merge
quick-but-needed-updated into master

In 2024-10-04: CiRunnersServiceCiRunnerJobsApdexSL... (gitlab-com/gl-infra/production#18667 - closed) EOC was wondering how to ssh into the runner-manager VMs, this low-effort MR removes dated info and adds a quick SSH section to help SRE quickly see inside said VMs.

Edited by Rehab

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