Docs: Add CLI reference section to docs/USAGE.md

[MozzamShahid]

File / lines: docs/USAGE.md CLI sections (lines 6-119); src/gmaps_scraper/cli.py lines 86-236

Problem: The usage guide has examples but no table of all flags. Important flags are not mentioned: --proxy / GMAPS_SCRAPER_PROXY, --timeout-ms, --settle-ms, --http-cookie-jar, --llm-env-file, --dump-debug-output, --retry-backoff-ms, --stagger-ms, etc.

Suggested fix:

1. Add a "CLI reference" section with a table of flags, defaults, and one-line descriptions.
2. Embed the output of uv run gmaps-scraper --help or keep it in sync manually.
3. Add short subsections for "Proxy, timeouts, and cookies" and "LLM setup".

Difficulty: good first issue
Impact: Medium — high value for operators and contributors.