docs: add cross-references between Code Mode server and domain-specific servers

Cloudflare has two categories of MCP servers across two repos:

- **Code Mode server** at `mcp.cloudflare.com` ([cloudflare/mcp](https://github.com/cloudflare/mcp)) -- 2 tools covering all ~2,500 API endpoints via code execution
- **Domain-specific servers** at `*.mcp.cloudflare.com` ([cloudflare/mcp-server-cloudflare](https://github.com/cloudflare/mcp-server-cloudflare)) -- 15 servers with curated, typed tools for specific products

Neither repo links prominently to the other. The root README here never mentions `mcp.cloudflare.com` or the Code Mode server. Users who find one repo don't know the other exists.

There should be:
1. A section in both READMEs explaining that both exist and when to use which
2. Guidance like: "Use the Code Mode server for broad API access across all products. Use the domain-specific servers for deeper, curated tooling in a particular product area."