fix valkey templating issues, add more docs

[jessebot]

This fixes the following when I try to deploy this chart via Argo CD:

Failed to load target state: failed to generate manifest for source 1 of 1: rpc error: code = Unknown desc = Manifest generation error (cached): failed to execute helm template command: failed to get command args to log: `helm template . --name-template pixelfed --namespace pixelfed --kube-version 1.31 --values /tmp/6f380b98-2c89-4328-8590-0e30dee6416f <api versions removed> --include-crds` failed exit status 1: Error: template: pixelfed/templates/deployment.yaml:199:31: executing "pixelfed/templates/deployment.yaml" at <.Values.valkey.primary.service.ports.valkey>: nil pointer evaluating interface {}.ports Use --debug flag to render out invalid YAML