Add docs on monitoring WAL failover

[rmloveland]

Fixes DOC-10268

Followup to #18511 with the monitoring piece

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Files changed:

• src/current/v24.1/cockroach-start.md

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[rmloveland]

@jbowens this is the content on WAL failover monitoring I'd like to add directly following merge of #18511

will be rebasing this PR on main once that other PR goes in and then the placement of this content will actually make sense (ideally)

opening in parallel for speed since v24.1 public release is next Monday

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[rmloveland]

@florence-crl wanted to add some context b/c this PR is "different", this is a followup to https://github.com/cockroachdb/docs/pull/18511 with just the monitoring info

I split it into a separate PR since that other one is taking a little time to get through review - that one will also be coming your way ASAP for review of course!