Updated README and docs for auth and Helm chart

The README now frames Hive as Tuist’s approach to shaping and building product with LLMs and agentic workflows, and documents how to configure Google and generic OIDC auth, the new HIVE_VISIBILITY setting, and how to deploy the generic Helm chart with or without the Tuist production overlay. The landing page at docs.hive.tuist.dev has also been updated to reflect the product positioning.