doc: Rotation the signer key for Rekor service

[osmman]

Depends on: #512

[openshift-ci]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[lance]

lgtm - though you might want to put a warning or note at the top that this procedure is incomplete and should not be done until the final step (updating TUF) is possible and documented.

[openshift-ci]

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