Add prominent documentation links to README and package

## Changes

### README.md
- Added new "Documentation" section after API Key setup
- Includes direct links to:
  - Main documentation site (pycancensus.readthedocs.io)
  - Getting Started tutorial
  - Working with Geographic Data tutorial
  - Example Gallery
  - API Reference
  - R to Python Migration Guide
- Makes documentation discoverable before Quick Start examples

### pycancensus/__init__.py
- Added documentation URL to package docstring
- Appears when users do `help(pycancensus)` in Python

### Note
- pyproject.toml already had Documentation URL in project.urls
- Documentation badge already present in README badges section

## Impact
Users can now easily find comprehensive documentation from:
- GitHub README (new prominent section)
- Python help() command
- PyPI package page (via pyproject.toml)
- README badge

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: dshkol

README.md

@@ -85,6 +85,17 @@ import pycancensus as pc
 pc.set_api_key("your_api_key_here")
 ```
 
+## Documentation
+
+**Full documentation is available at [pycancensus.readthedocs.io](https://pycancensus.readthedocs.io/)**
+
+The documentation includes:
+- **[Getting Started Tutorial](https://pycancensus.readthedocs.io/en/latest/tutorials/getting_started.html)** - Learn the basics
+- **[Working with Geographic Data](https://pycancensus.readthedocs.io/en/latest/tutorials/working_with_geometry.html)** - Maps and spatial analysis
+- **[Example Gallery](https://pycancensus.readthedocs.io/en/latest/auto_examples/index.html)** - Real-world usage examples
+- **[API Reference](https://pycancensus.readthedocs.io/en/latest/api/index.html)** - Complete function documentation
+- **[R to Python Migration Guide](https://pycancensus.readthedocs.io/en/latest/migration.html)** - For R cancensus users
+
 ## Quick Start
 
 ```python

pycancensus/__init__.py

@@ -3,6 +3,8 @@
 
 This package provides integrated, convenient, and uniform access to Canadian
 Census data and geography retrieved using the CensusMapper API.
+
+Documentation: https://pycancensus.readthedocs.io/
 """
 
 __version__ = "0.1.0"