docs: add strict CSP starting example

[rajanpanth]

Problem

New django-csp users need a safe starting point for a strict CSP. The maintainers asked for a base configuration in the documentation rather than changing the library default policy (see #156).

Closes #156.

Solution

Add a report-only strict-policy example that:

• Uses the project-provided NONE, NONCE, SELF, and STRICT_DYNAMIC constants.
• Explains why it must be rolled out in report-only mode first.
• Links the required nonce middleware/template setup.
• Warns against weakening the policy with unsafe-inline or unsafe-eval merely to suppress reports.

The example intentionally does not prescribe a universal enforced policy; users add resource-specific directives after reviewing their own violations.

Testing

• sphinx-build -W -b html docs <temporary output directory>