document CSP explicitly

The existing CSP docs list the specific attributes that should be included, which is a good overview of what's necessary. However, for someone setting up the integration, there's still a lot of work required to port that to their specific config. Let's offer some quick config snippets for common platforms:

• nginx
• traefik, via middleware
• caddy
• cloudflare

It's also worth noting that given the requirement for CSP to be served via headers, popular static-site hosting like Github Pages cannot work, because GHP does not allow header configuration AFAIK. If a site-operator fronts a GHP site with e.g. Cloudflare, or some other reverse proxy within their control, then CSP headers are configurable, and the website should be validated.

Related but separate is the ask for CSP-check-in-code.

Start with the existing CSP docs linked in the issue and review how the listed attributes map to configuration on nginx, Traefik middleware, Caddy, and Cloudflare. Add concise snippets for those platforms and document the GitHub Pages header limitation, including the reverse-proxy case; the work is done when an integration operator can configure and validate CSP from the documentation.