docs: deprecate .config.prowgen in favor of ci-operator config

[Prucek]

Summary

• Document deprecation of .config.prowgen file in favor of ci-operator config fields
• Add prowgen stanza documentation to ci-operator reference (private, expose, disable_rehearsals, enable_secrets_store_csi_driver)
• Document new per-test fields: slack_reporter, disable_rehearsal, max_concurrency
• Update private-repositories, contributing-openshift-release, and notification docs to use new config approach
• All .config.prowgen references now point to deprecation notices with migration guidance

Companion PR: openshift/ci-tools#5012

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[openshift-ci]

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[Prucek]

/hold

[openshift-ci]

@Prucek: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/links	175b3bb	link	true	/test links

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