docs: comprehensive README rewrite to reflect multi-region architecture

Completely rewrote the README to accurately document the current production
deployment of coderdemo.io with hub-and-spoke architecture across three regions.

## Major Changes

### Architecture Documentation
- Add clear hub-and-spoke architecture explanation
- Include ASCII diagram showing 3-region topology
- Detail hub region (us-east-2) vs spoke regions (us-west-2, eu-west-2)
- Clarify which infrastructure is repeatable vs non-repeatable

### Updated Repeatability Warning
- Clarify us-east-2 as non-repeatable hub (database, terraform backend, VPC)
- Identify eu-west-2 as clean repeatable spoke template for new regions
- Note us-west-2 as hybrid deployment with both server and proxy

### Comprehensive Deployment Guide
- Step-by-step hub region deployment (foundation layer)
- Repeatable spoke region deployment process
- Correct dependency order for Kubernetes applications
- Real-world configuration examples

### New Sections
- Multi-region architecture details (database strategy, proxy strategy, networking)
- Security considerations (secrets management, network security, IAM)
- Cost optimization strategies (Karpenter, Aurora Serverless v2, fck-nat)
- Troubleshooting guide with common issues and solutions
- Configuration examples for terraform.tfvars and backend.hcl

### Technical Accuracy Updates
- Aurora Serverless v2 PostgreSQL (not generic RDS)
- AWS ACM for SSL/TLS certificates (removed all Cloudflare references)
- VPC peering for cross-region database connectivity
- GitHub OAuth integration details
- Karpenter autoscaling configuration

### Documentation Structure
- Better organization with clear sections
- Links to detailed docs (MULTI_REGION_DEPLOYMENT.md, INFRASTRUCTURE_BEST_PRACTICES.md)
- Practical examples and commands
- Prerequisites and version requirements

This README now serves as a complete reference for deploying and
understanding the coderdemo.io infrastructure.

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

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

Author: Noah Boyers