Update the public documentation to show how to enable TLS using user secrets

[DnPlas]

Context

The current public documentation shows how to enable TLS using actions, a method that will be replaced by user secrets. The documentation should reflect these changes.

This documentation change is a result of #398, #402 and #401.

What needs to get done

Update the documentation for configuring the TLS with user secrets. This documentation should include:

1. How to add a user secret that istio-pilot can use for configuring TLS
2. The expected juju version
3. What are other configurations that are required (oidc and dex's public-url)
4. References to the juju migration guide for the people that need migration steps.

Definition of Done

A public entry in the charmed-kubeflow.io page with the above instructions.

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The internal ticket has been created: https://warthogs.atlassian.net/browse/KF-5513.

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