docs: add Sphinx documentation site with user and contributor guides

[andygrove]

Summary

Add a Sphinx-based documentation site under docs/, mirroring the layout used by sibling DataFusion subprojects (datafusion, datafusion-comet, datafusion-python).

What changed

• New docs/ directory with Sphinx config (source/conf.py), MyST parser for Markdown, pydata-sphinx-theme, and build.sh/Makefile/make.bat scaffolding.
• Landing page (docs/source/index.md) with the Quickstart example and three toctrees (Links, User Guide, Contributor Guide).
• User guide (docs/source/user-guide/): installation, quickstart walkthrough, SessionContext lifecycle and threading, DataFrame + SQL API, Parquet ingestion with ParquetReadOptions, project status checklists.
• Contributor guide (docs/source/contributor-guide/): build & test workflow, code style (spotless, cargo fmt/clippy, license headers), DataFusion / protobuf version bump recipe, releasing placeholder.
• Trim root README.md to a slim Quickstart + pointers into the docs site.
• Add a slim CONTRIBUTING.md pointing to the contributor guide.
• .gitignore excludes docs/build/ and docs/venv/.

Build

cd docs
python3 -m venv venv
source venv/bin/activate
pip install -r requirements.txt
./build.sh

build.sh uses -W so any future Sphinx warning fails the build. Site builds cleanly today with zero warnings.

Not in this PR

• Javadoc / API reference generation.
• GitHub Actions publish workflow / ASF site target.
• Logo asset and visual branding.
• Multi-version docs.