doc: Document new CONTENT_SECURITY_POLICY_REPORT_ONLY env var

[frojd-trond-froding]

Also add documentation for the existing CONTENT_SECURITY_POLICY env var

Related open-webui pull request that adds the new reports only env var open-webui/open-webui#23575

[pr-validator-bot]

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[frojd-trond-froding]

Now the code in the pull request is in the dev branch (see open-webui/open-webui#23575 (comment)) so I think one could merge this now.