Add how-to guide about securing a charm

As an organisation, we need to make compliance requirements clearer for our stores. It would be helpful to have a how-to guide called "How to secure a charm" or similar, that charmers can reference as part of their compliance effort.

We currently have [Good practices](https://github.com/canonical/operator/pull/2655) in the Security explanation. It would be more discoverable to have a dedicated how-to guide.