Add comprehensive PyPI publishing guides

Two guides for publishing to PyPI:

1. PUBLISHING.md - Comprehensive guide covering:
   - PyPI account setup with 2FA and API tokens
   - Build tools installation
   - Pre-publication checklist
   - Step-by-step build and upload process
   - Testing with TestPyPI first
   - Production PyPI publication
   - Post-publication verification
   - GitHub release integration
   - Automated publishing with GitHub Actions
   - Troubleshooting common issues
   - Security best practices

2. PUBLISH_NOW.md - Quick-start guide with:
   - Streamlined steps for immediate publication
   - Copy-paste commands ready to run
   - Testing workflow (TestPyPI → PyPI)
   - Verification steps
   - Post-publication updates

After following these guides, pycancensus will be installable via:
- pip install pycancensus
- uv pip install pycancensus
- poetry add pycancensus
- pipenv install pycancensus

Author: dshkol