[kong] improve RBAC documentation

[rainest]

What this PR does / why we need it:

Improves existing RBAC docs to better show how to set up the initial super-admin and allow the controller to talk to RBAC-secured instances.

Special notes for your reviewer:

• This does not discuss readiness/liveness checks, which also currently require tokens. Intent is to add a custom server block listen to approximate status_listen (and eventually replace it with the standard status_listen), removing the need for admin API status calls. This will be a separate PR.
• ed7e1b6 further adds an example for the controller within values.yaml. None for Kong itself, since that variable should only be in place temporarily.

Checklist

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• PR is based off the current tip of the next branch and targets next, not master
• Title of the PR and commit headers start with chart name (e.g. [kong])

[hbagdi]

This does not discuss readiness/liveness checks, which also currently require tokens. Intent is to add a custom server block listen to approximate status_listen (and eventually replace it with the standard status_listen), removing the need for admin API status calls. This will be a separate PR.

I might be missing something. Doesn't the existing server block configmap and metrics take care of the health checks? We don't rely on Admin API status checks.