docs: add TOC, package links, and required/optional env column

[amrtgaber]

Summary

Knocks out the three open README housekeeping issues in one PR.

Closes #13 — adds a Table of Contents with anchor links to every top-level section and key subsections.

Closes #14 — links each row in the Tech Stack table to the canonical package docs (FastAPI, PostgreSQL, asyncpg, SQLAlchemy, Alembic, FastAPI-Users, slowapi, uv, Docker, Docker Compose, Pytest, pytest-asyncio, Ruff, pre-commit).

Closes #15 — adds a "Required" column to the Environment Variables table.

Required/Optional semantics

Every env var in app/config.py has a default value, so strictly speaking nothing is required to start the app in dev. The "Required" column instead reflects what production config validation (Settings.validate_production_settings) enforces:

• SECRET_KEY — rejects weak defaults and values shorter than 32 chars
• DATABASE_URL — rejects the default postgres:postgres@ credentials
• CORS_ORIGINS — empty in prod means the frontend can't talk to the backend (dev uses localhost:5100-5199 regardless of this var)

These three are marked Required; everything else is Optional. The table has an intro sentence explaining the semantics so readers don't need to dig into config.py.

Also reordered the table so ENVIRONMENT appears first, since the "Required" semantics depend on it.

Test plan

• uv run ruff format --check . — clean
• Verify all TOC anchor links resolve correctly when rendered on GitHub
• Verify all Tech Stack package links are still valid