Add AGENTS.md and enrich __init__.py module docstring

Add python/datafusion/AGENTS.md as a comprehensive DataFrame API guide
for AI agents and users. It ships with pip automatically (Maturin includes
everything under python-source = "python"). Covers core abstractions,
import conventions, data loading, all DataFrame operations, expression
building, a SQL-to-DataFrame reference table, common pitfalls, idiomatic
patterns, and a categorized function index.

Enrich the __init__.py module docstring from 2 lines to a full overview
with core abstractions, a quick-start example, and a pointer to AGENTS.md.

Closes #1394 (PR 1a)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: timsaucer