patch(DPE-9692): Documentation for encryption at rest

[Gu1nness]

🏷️ Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Tooling and CI
• Dependencies upgrade or change
• Chores / refactoring

📝 Description

Documentation for encryption at rest.
Must be merged AFTER we have released the charms and set the right revision numbers.

🧪 Manual testing steps

1. cd docs
2. make serve

🔬 Automated testing steps

✅ Checklist

• My code follows the code style of this project.
• I have added or updated any relevant documentation.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.