Add CSP to health checks article

[erikjanwestendorp]

What type of issue is it?

Missing documentation

What article/section is this about?

(https://docs.umbraco.com/umbraco-cms/extending/health-check/guides)

Describe the issue

It would be nice to add an article about the CSP health check (When umbraco/Umbraco-CMS#16830 is merged). See comment: umbraco/Umbraco-CMS#16830 (comment)

[sofietoft]

Thanks for adding this issue, so we wont forget that this indeed needs to be documented when it's released 🤩 💪