Convert documentation from reStructuredText to Markdown

[laughingman7743]

Summary

Convert all .rst files under docs/ to Markdown (.md) format using MyST-Parser, following the README conversion done in #650.

Motivation

• README was already converted from RST to Markdown in Add project logo to README and documentation #650
• Markdown is more widely familiar and easier to maintain
• Consistent format across the project (README + docs)

Scope

25 .rst files to convert:

Root docs/ (13 files)

• index.rst
• introduction.rst
• usage.rst
• cursor.rst
• pandas.rst
• arrow.rst
• polars.rst
• s3fs.rst
• spark.rst
• sqlalchemy.rst
• null_handling.rst
• testing.rst
• api.rst

docs/api/ (12 files)

• connection.rst, converters.rst, errors.rst, models.rst, utilities.rst
• pandas.rst, arrow.rst, polars.rst, s3fs.rst, spark.rst
• filesystem.rst, sqlalchemy.rst

Approach

• Add MyST-Parser as a Sphinx extension to support Markdown in Sphinx
• Convert each .rst file to .md preserving all content and cross-references
• Update docs/conf.py extensions and any toctree references
• Verify documentation builds correctly with make docs