chore(backend): Add documentation for TLS cert rotation

[alyssacgoins]

Chore description

When pod-to-pod TLS (option recently added in #12082) is enabled, certs must be renewed several times per year. When secrets are updated or renewed, the user will need to restart their cluster. This is currently not documented and is not obvious to the user.
Documentation explaining the TLS cert renewal process, including successfully applying the certs and restarting the cluster, should be added in the backend README.

Labels

/area backend

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[aniketpati1121]

Nice work! Maybe we could also include an example command sequence for renewing the certs and restarting the cluster to make it even clearer for users.