Docs: Add basic Redis troubleshooting

What does this MR do?

In a large GitLab Premium customer's internal support case we encountered a Couldn't connect to redis instance error and wondered how to start troubleshooting it. We currently don't have relevant issues about this error or how to check the Redis connection from GitLab's side.

This is a first idea, although it could of course be a GL-independent network problem.

Leaving it in draft mode, until we have confirmation by the customer that the idea is useful.

Related issues

Author's checklist

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If you are only adding documentation, do not add any of the following labels:

• ~"type::feature"
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• ~"backend"
• ~"type::bug"
• ~"database"

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