Add comprehensive GKE deployment guide for GCP OAuth

This guide provides step-by-step instructions for deploying Headlamp
with GCP OAuth authentication on Google Kubernetes Engine (GKE).

## What's Included

- Complete setup from OAuth credentials to working deployment
- Environment variable configuration with Kubernetes Secrets
- LoadBalancer service configuration
- RBAC examples for GCP user authentication

## RBAC Examples

The guide includes 5 detailed RBAC configuration examples:

1. Single user with cluster-admin role
2. Multiple users with read-only view role
3. Namespace-specific edit permissions
4. Custom role for limited resource access
5. Google Group-based permissions for Google Workspace

## Key Features

- Kubernetes Secret management for OAuth credentials
- nip.io DNS service for IP-based access (workaround for Google OAuth IP restrictions)
- RBAC verification commands
- Complete troubleshooting section
- Security best practices
- Full deployment example with all components

## Configuration Reference

Documents all required environment variables:
- HEADLAMP_CONFIG_GCP_OAUTH_ENABLED
- HEADLAMP_CONFIG_GCP_CLIENT_ID
- HEADLAMP_CONFIG_GCP_CLIENT_SECRET
- HEADLAMP_CONFIG_GCP_REDIRECT_URL

This guide complements the existing setup documentation and provides
GKE-specific deployment instructions with practical examples.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: itpick