[docs:gsoc26] Documentation for X.509 Certificate generator templates

This issue tracks documentation for the GSoC 2026 X.509 Certificate Generator Templates project.

Related implementation issues:

1. https://github.com/openwisp/openwisp-controller/issues/1356
2. https://github.com/openwisp/openwisp-controller/issues/1357
3. https://github.com/openwisp/openwisp-controller/issues/1358
4. https://github.com/openwisp/openwisp-controller/issues/1359
5. https://github.com/openwisp/openwisp-controller/issues/1360
6. https://github.com/openwisp/openwisp-controller/issues/1361
7. https://github.com/openwisp/openwisp-controller/issues/1377
8. https://github.com/openwisp/django-x509/issues/222

Documentation deliverables from the GSoC proposal

Create a dedicated documentation page for certificate templates covering:

• Overview of what certificate templates are and when to use them.
• Use cases listed in the proposal: HTTPS, 802.1x, mTLS, and device authentication.
• Step-by-step instructions for creating a Certificate Authority.
• Step-by-step instructions for creating a blueprint certificate.
• Step-by-step instructions for creating a certificate template.
• Step-by-step instructions for assigning the template to a device.
• Step-by-step instructions for using certificate variables in configuration templates.
• Template variable reference for certificate PEM, private key PEM, and certificate UUID.
• Lifecycle behavior on assignment, unassignment, renewal, and device property changes.
• Explanation of configurable regeneration behavior after that behavior is decided during implementation.
• Supported options and limitations, including the proposal's out-of-scope items: SAN, OCSP, and Let's Encrypt.
• A short demonstration video.

Static media files, such as images or GIFs, should be added to the docs branch after the implementation of each feature, so the final workflow and functionality can be documented visually.