Add Docling parser adapter (table extraction)

[kiyeonjeon21]

What

Docling (IBM) has 97.9% table extraction accuracy. The adapter exists at src/parsemux/parsers/docling.py but is untested with real Docling installation.

How

1. pip install parsemux[docling]
2. Test with table-heavy PDFs
3. Verify DoclingParser.is_available() works
4. Add integration tests

Context

Docling is especially valuable for financial reports, invoices, and academic papers with complex tables.