RFE-9862: Document Podman default subnet overlap on RHCOS nodes

[DeokarT]

RFE-9862. Same collision as OCPBUGS-9296: oc debug plus a plain podman run brings up 10.88.0.0/16, and that route can stay after the container is gone.

This PR only documents the workaround. It does not change the default CIDR in MCO.

• New procedure on Gathering data about your cluster: --network host / --network none, delete leftover podman0 / cni-podman0, optional MachineConfig drop-in for default_subnet.
• Short warnings on RHCOS features, Agent installer networking, toolbox, and oc debug host access.

Test plan

• Preview Gathering data about your cluster and check the new "Running Podman on a cluster node" section after toolbox.
• Preview RHCOS key features: IMPORTANT under container tools.
• Preview Agent-based installer networking: extra sentence on the 10.88 note.
• Preview command-line host access: IMPORTANT after chroot /host.

Related: https://redhat.atlassian.net/browse/RFE-9862, https://redhat.atlassian.net/browse/OCPBUGS-9296, https://redhat.atlassian.net/browse/RHEL-34282, https://access.redhat.com/solutions/6961655, podman-container-tools/podman#17847

[openshift-ci-robot]

@DeokarT: This pull request references RFE-9862 which is a valid jira issue.

Warning: The referenced jira issue has an invalid target version for the target branch this PR targets: expected the feature request to target the "5.1.0" version, but no target version was set.

Details

In response to this:

RFE-9862. Same collision as OCPBUGS-9296: oc debug plus a plain podman run brings up 10.88.0.0/16, and that route can stay after the container is gone.

This PR only documents the workaround. It does not change the default CIDR in MCO.

• New procedure on Gathering data about your cluster: --network host / --network none, delete leftover podman0 / cni-podman0, optional MachineConfig drop-in for default_subnet.
• Short warnings on RHCOS features, Agent installer networking, toolbox, and oc debug host access.

Test plan

• Preview Gathering data about your cluster and check the new "Running Podman on a cluster node" section after toolbox.
• Preview RHCOS key features: IMPORTANT under container tools.
• Preview Agent-based installer networking: extra sentence on the 10.88 note.
• Preview command-line host access: IMPORTANT after chroot /host.

Related: https://redhat.atlassian.net/browse/RFE-9862, https://redhat.atlassian.net/browse/OCPBUGS-9296, https://redhat.atlassian.net/browse/RHEL-34282, https://access.redhat.com/solutions/6961655, podman-container-tools/podman#17847

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[openshift-ci]

Hi @DeokarT. Thanks for your PR.

I'm waiting for a openshift member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

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