Add missing detail about introduction check subcommand

What does this MR do?

[1] Fix link and details about Omnibus praefect check command.

I don't see a MR in 14.6 that would account for this change, but 14.5 has: omnibus-gitlab!5688 (merged) and relates to gitaly!3989 (merged) both are 14.5

[2] Add mention of the NTP check. This is a first iteration, as I've not documented what to do when it fails.

We have a ticket where this check is failing:

checking with NTP service at and allowed clock drift 60000 ms [correlation_id: 01G2EZGMMKGEG4T90M1K36QC4V]
Failed (fatal) error: gitaly node at tcp://hostname:8075: rpc error: code = DeadlineExceeded desc = context deadline exceeded

But I'm 🌴 this week and so won't be able to add more details until we get to the bottom of it.

Related issues

omnibus-gitlab#6472 (closed)

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