Doc: howto SSL/TLS for RGW

We currently have reference-level documentation on how to enable SSL/TLS for Ceph Rados Gateway (RGW), but it is terse and assumes prior knowledge. This makes it difficult for users to set up a secure RGW endpoint confidently.

To address this, we propose creating a dedicated How-To guide for enabling SSL/TLS for the RGW component in MicroCeph, following the [Diataxis documentation framework](https://diataxis.fr/how-to-guides/). The focus should be on practical, step-by-step instructions that users can follow with minimal assumptions, and that cover:

* Required preconditions (e.g. presence of RGW, valid certificates, MicroCeph deployment)
* Options for obtaining or generating TLS certificates (self-signed vs Lets Encrypt vs external CA)
* Configuration steps for MicroCeph to enable and verify SSL/TLS on RGW
* How to check that HTTPS is correctly set up and used
* Troubleshooting common issues (e.g. certificate mismatch, browser warnings, config key mistakes)

The guide should use concrete MicroCeph examples and commands.

The guide should be consistent with the existing documentation here: https://canonical-microceph.readthedocs-hosted.com/stable/