Self Hosted config options (helmet's adaptation)

### Describe the Request

I think it would be nice to add at least:

OR
- a more precise docs for CONTENT_SECURITY_POLICY_*
- a doc page for how to setup livepreview for self hosted

because now (not some times ago) I had a CSP error with IFRAME.

So I spend hours to understand (not find it but I deducted it) that

```CONTENT_SECURITY_POLICY_DIRECTIVES__FRAME_SRC="array:https://www.yourwebsite.com/"```

In the .env file was the solution ^^

### Maintainence Strategy

As often as the config option of CSP will change