Gitaly doc revisions - network time, praefect replica paths, GC server hooks

What does this MR do?

NTP

Gitaly cluster gained a dependency on NTP in 15.4 which (by default) doesn't work behind firewalls and in airgapped environments. Discussed in a related MR and it came up in a customer ticket (internal link)

It might get removed in %15.6 and / or backported - gitaly!5040 (merged) but that's in the future.

While documenting the NTP requirement for Gitaly Cluster, Geo has a soft dependency on pool.ntp.org as well, so I'm linking to the new documentation on that topic

Replica paths

Gitaly cluster no longer uses @hashed storage paths for new repos. This needs noting in the upgrade notes because it affects the deployment of server hooks, even though that's not documented for Gitaly Cluster (Gitaly Cluster Docs: clarify server hooks (gitaly#4202))

Gitaly cluster server hooks

Started documenting it as the Praefect replica path thing needs a mention now. Not enough to sort out Gitaly Cluster Docs: clarify server hooks (gitaly#4202)

Related issues

NTP

• gitaly#4598 (closed)
• Geo rake check: we cannot assume pool.ntp.org i... (#381422 - closed)

Replica paths / Server hooks

• gitaly#4218 (comment 1164877849)
• gitaly#4202

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