docs: Convert Sphinx-style docstrings to NumPy-style

Convert all :param/:return:/:raises: docstrings to NumPy-style format
as specified in CONTRIBUTING.md. This ensures proper rendering in
API documentation generated by mkdocstrings.

Files updated:
- expression.py: 28 methods converted
- table.py: 24 methods/functions converted
- heading.py: 2 methods converted
- utils.py: 8 functions converted

Also removes unused parse_sql() function.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dimitri-yatsenko