pipdeptree's documentation lived entirely in README.md, limiting
discoverability and making it hard to organize growing content like
output format examples and CLI reference.

Sphinx with furo theme mirrors the virtualenv project's docs setup and
enables ReadTheDocs hosting. All examples use real pipdeptree output
from the dev environment. sphinx-argparse-cli auto-generates the CLI
reference from the argparse parser so it stays in sync with the code.

Also fixes mermaid output to use <br/> instead of \n for node label
line breaks, since Mermaid's JS renderer treats \n as literal text.