pipdeptree's documentation has lived entirely in README.md, which made it hard to organize growing content like output format examples, CLI reference, and usage patterns into a discoverable structure.

📝 This adds a full Sphinx documentation site using the furo theme, mirroring the virtualenv project's docs setup. The site follows the Diataxis framework with tutorial, how-to guides, reference, and explanation sections. Every pipdeptree command example shows real output from the dev environment so nothing is fabricated. The CLI reference uses sphinx-argparse-cli to auto-generate from the argparse parser, keeping it permanently in sync with the code. Mermaid diagrams throughout visualize dependency graphs, conflict detection, and the internal architecture.

🔧 Build infrastructure includes a tox -e docs environment with sphinx-build -W (warnings as errors), a .readthedocs.yml for RTD hosting, and docs added to the CI matrix. Also fixes the mermaid renderer to use <br/> instead of \n for node label line breaks, since Mermaid's JS renderer treats \n as literal text.