docs: Separate Gitaly and Praefect docs to prepare for Raft

Problem to solve

Because introducing Raft (&13562) will create an alternative way to create a Gitaly Cluster, we can no longer assume that Praefect will be in play in every Gitaly Cluster, and we also need to continue to support folks that don't cluster at all.

Further details

It will be difficult to introduce new documentation for a Raft-based Gitaly Cluster with the documentation for Gitaly and Praefect intertwined as it is currently. We need to create greater distinction between Gitaly without Praefect, and Gitaly Cluster (with Praefect).

Each type of Gitaly installation needs its own set of task-based configuration, maintaining, monitoring, and troubleshooting documentation that is separate from the other. It's no longer desirable to have these combined for the sake of efficiency and keeping the documentation DRY (not repeating ourselves).

Proposal

Work through the complete set of Gitaly and Praefect documentation at https://docs.gitlab.com/administration/gitaly/ and:

• Separate out documentation of Gitaly by itself and Gitaly with Praefect.
• Make it clear that all current Gitaly Cluster documentation refers to Gitaly Cluster with Praefect so that it's easier to disambiguate future documentation for Gitaly Cluster with Raft.

Who can address the issue

@eread, the assigned technical writer for groupgitaly