Update SIG Node CI test coverage guide

[natasha41575]

Here is a proposal for how we can reorganize the SIG Node CI testgrid to make tests easier to find. I generally find it hard to navigate the current testgrid; it is not obvious (at least to me) where or if a particular test is running in the testgrid today.

This came out of me picking up kubernetes/kubernetes#134862; while starting to look at what we had previously defined as our test coverage guidance (which is still not implemented), I thought the current proposal can be improved. This is intended to be an incremental improvement, and can be iterated upon in the future. I also want to be careful not to remove any existing test coverage.

Also another, note, there are some highly specialized test tabs like sig-node-problem-detector, sig-node-security-profiles-operator, sig-node-ec2, sig-node-gpu etc, I don't intend to touch those.

All feedback and suggestions are welcome. I am not an expert on test-infra or all node features so please feel free to bring up issues or mistakes :)

/sig node

/assign @SergeyKanzhelev
/cc @mariafromano-25 @haircommander @kannon92

/hold
just to gate merging until we are ready

[k8s-ci-robot]

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