Convert documentation from RST to Markdown

[laughingman7743]

Summary

• Convert 13 root documentation files under docs/ from RST to MyST Markdown (.md) format
• Keep 12 API reference files as .rst since autodoc generates RST internally
• Improve index page with Quick Start section and dropdown TOC navigation using sphinx-design

Closes #652

Details

Markdown conversion (13 root docs)

Files converted: index, api, testing, introduction, cursor, spark, arrow, polars, s3fs, pandas, usage, null_handling, sqlalchemy

Conversion patterns applied:

• RST labels (.. _label:) → MyST labels ((label)=)
• RST directives (.. code::, .. note::, .. toctree::) → MyST directives (```{directive})
• RST grid tables and list-table → standard Markdown tables
• RST cross-references (:ref:) → MyST {ref} syntax
• RST hyperlink references → inline Markdown links
• RST definition lists → MyST definition list syntax (via deflist extension)

API docs kept as RST (12 files)

autodoc generates RST output internally (.. py:class::, .. py:function::, etc.). Using MyST's {autoclass} syntax caused these directives to render as raw text. Rather than wrapping every directive in {eval-rst} blocks, the API reference files are kept as .rst. Sphinx handles .rst/.md coexistence via the source_suffix configuration.

Index page improvements

• Added Quick Start section with install command and example code
• Added sphinx-design for dropdown directive support
• Organized toctree into categorized dropdown sections: Getting Started, Cursors, Integrations, Advanced Topics, API Reference
• Sidebar now shows grouped navigation with section headings

Configuration changes

• pyproject.toml: Added myst-parser and sphinx-design to dev dependencies
• docs/conf.py: Added MyST-Parser extension with colon_fence, fieldlist, deflist extensions; added sphinx_design extension

Test plan

• sphinx-build -b html docs docs/_build/html builds successfully
• All pages render correctly (root docs as Markdown, API docs as RST)
• API documentation renders properly with class/function signatures
• Cross-references between files resolve properly
• Code blocks are syntax-highlighted
• Tables render properly
• Toctree navigation and dropdown TOC work
• Sidebar shows grouped navigation
• make chk passes (lint, format, type checks)

🤖 Generated with Claude Code