Generic Helm chart and self-hosting documentation

Self-hosters can now deploy Hive with a generic Helm chart that no longer includes Tuist-specific assumptions. The default values are provider-neutral, use HIVE_VISIBILITY=public, and require the operator to supply their own host, image pull secret, and secrets. Tuist’s own production configuration is preserved in a separate values-production.yaml overlay. The README has been rewritten to document the generic deployment, Google and OIDC authentication, and the new visibility option.