docs: Add guide on how to debug CustomStateMetrics feature quickly

[CatherineF-dev]

What this PR does / why we need it:

More and more bugs are opened for CustomStateMetrics feature. This will help users to debug CustomStateMetrics by themselves quicker.

How does this change affect the cardinality of KSM: (increases, decreases or does not change cardinality)

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

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This pull-request has been approved by: CatherineF-dev
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[dashpole]

/assign @rexagod
/triage accepted

[rexagod]

The change seems fairly straight-forward. I'd suggest adding a section demonstrating the usage of https://github.com/go-delve/delve to catch contexts at breakpoints set by the user.

[CatherineF-dev]

Could you add how to use some debugging tools in another PR? I don't use delve recently

Not everyone knows minikube. Could we merge this?

[dgrisonnet]

Instead of adding it to the README, I think it would be better to put it in its own doc under https://github.com/kubernetes/kube-state-metrics/tree/main/docs, or maybe in a new directory there.

I suggested reworking the structure of our doc directory in the past, we might want to revive that effort.

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[CatherineF-dev]

/remove-lifecycle stale