feat: add kratos webhook header allowlist config details

[K3das]

Adds Kratos webhook header allowlist configuration details

Related Issue or Design Document

ory/kratos#4309

Checklist

• I have read the contributing guidelines and signed the CLA.
• I have referenced an issue containing the design document if my change introduces a new feature.
• I have read the security policy.
• I confirm that this pull request does not address a security vulnerability.
  If this pull request addresses a security vulnerability,
  I confirm that I got approval (please contact [email protected]) from the maintainers to push the changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added the necessary documentation within the code base (if appropriate).

Further comments

@aeneasr mentioned to update the advanced configuration docs (ory/kratos#4309 (comment)) but I'm not really sure they're relevant there.

[vinckr]

thanks 🐝

lgtm, not sure when to merge though

[K3das]

should probably be merged with next Kratos release