Add `Document Policy` to security section

[summercms]

Feature Policy is deprecated and has been replaced with Permissions Policy and Document Policy.

This repo has Permissions Policy found here: https://github.com/h5bp/server-configs-apache/blob/main/h5bp/security/permissions-policy.conf

It should also contain Document Policy.

Link: https://wicg.github.io/document-policy/

[LeoColomb]

Document-Policy header is not yet an official W3C standard, not even an official working draft.
Its usage is not documented on MDN either.
Let's keep Document-Policy integration on-hold while it stabilizes.

[summercms]

Added a request with MDN: mdn/mdn#394

Happy to see a hold label, as I meant this to be added as a future update.

Would be nice if someone could add an example, so we can do some testing.