Docs: Clarify development style guide for Certificate secrets

Summary

We should clarify the difference and expectations of Secrets containing certificates.

https://docs.gitlab.com/charts/development/style_guide.html#certificates does not provide enough clarification.

Details

Quoting the docs in their current state:

registry:
  certificate:
    secret: <secret name>
    key: <key name inside the secret to fetch>

Except, in many cases, we're explicitly expect a Secret of type: kubernetes.io/tls as made by kubectl create secret tls ... in place of kubectl create secret genertic ...

Why? This particular certificate is not a TLS/SSL certificate, but rather the JWT signing certificate's public content that the Registry chart loads, so that it can verify the authentication payloads from the GitLab API.

There's a distinction between secret types, and we should ensure each is clearly documented.