Add fastgen-cli to Developer Tools

[YIbaikaishui]

What

Adds fastgen-cli to the Developer Tools section (appended at the end, per contributing.md).

Why it's awesome

• Solves FastAPI's structural drift. FastAPI is famously unopinionated; projects drift into scattered routers and unknown modules. fastgen make module <feature> scaffolds one vertical-slice module per folder (domain / application / infrastructure / api + tests) and auto-mounts its router into main.py — no hand-editing, no config file.
• Auto-maintained registry. <src>/modules/__init__.py maps every module to its import path, so both humans and AI agents can see the whole project structure at a glance (fastgen list).
• Zero runtime lock-in. Everything generated is plain FastAPI + SQLAlchemy; nothing requires fastgen at runtime. You can drop the tool anytime and keep an ordinary project.
• Best-practice baseline in one command. fastgen new ships src/-layout, pydantic-settings config, async SQLAlchemy session, Alembic migrations, and a test suite.
• Written in Rust, distributed like uv. Single self-contained binary on PyPI (pip install fastgen-cli) — wheels for Linux/macOS/Windows, zero dependencies, no Python runtime needed to run the tool.