Add documentation for the django-framework for the config option `django-secret-key-id`. --- - [X] I've followed the [contribution guidelines](https://github.com/canonical/charmcraft/blob/main/CONTRIBUTING.md). - [X] I've signed the [CLA](http://www.ubuntu.com/legal/contributors/). - [X] I've successfully run `make lint && make test`. - [X] I've added or updated any relevant documentation. - [ ] In documents I changed, I [added a meta description](https://canonical-starflow.readthedocs-hosted.com/how-to/add-a-page-meta-description/) if one was missing. - [ ] I've updated the relevant release notes.