Add docs about Registry deployment via Operator

This change adds how to use the Operator to deploy a Registry Server
instance. Specifically, documentation under
`docs/toolhive/guides-registry` only mentions that you can use the
Operator to deploy a Registry Server instance and links back to a page
under `docs/toolhive/guides-k8s` containing all the documentation.

Documentation about using Postgres password file is not there because
we're working to make that transparent to the User.

I left documentation using plain Kubernetes resources untouched in the
`docs/toolhive/guides-registry` guide.

Author: blkt