CON-12531 Add DOKS multi-registries documentation

[llDrLove]

Add the multi-registries DOKS documentation for the POST and DELETE api calls. For now, this is only exposed using the public api and not integrated into the go, python and ruby clients.

[DO-rrao]

Hey,
I have been able to add multiple registries successfully.
However, I am not able to hit the endpoint - Remove Container Registries from Kubernetes Clusters
Can you send a screenshot with a 200 response for this endpoint?
otherwise, LGTM!

[llDrLove]

Hi @DO-rrao , here is a command I've ran that outputs the http code (204 success) with a successful deletion of the registries attached to a cluster.

> curl -X DELETE \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer <redacted>" \
  -d '{"cluster_uuids": ["3f538b59-9d57-412b-a72c-267969144e1d"], "registries": ["another-registry-001", "olove-other-registry"]}' -w "%{http_code}\n" \
  "https://api.digitalocean.com/v2/kubernetes/registries"

Output: 204

As you can see, this is the production api url using the curl command example added in this PR which results in 204 response from the server.