Document all available settings and environment variables

[jlowin]

FastMCP has a rich Settings class with many configurable options (FASTMCP_HOST, FASTMCP_PORT, FASTMCP_LOG_LEVEL, FASTMCP_TRANSPORT, etc.) that are automatically available as environment variables via pydantic-settings, but there's no dedicated documentation page listing them all.

Users currently have to read the source (src/fastmcp/settings.py) to discover what's configurable. A settings reference page would make these much more discoverable — especially for deployment scenarios where env vars are the primary configuration mechanism (Docker, ECS, K8s).