Convert docs to sphinx

docs/development.rst

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+Development
+===========
+
+The repo-local ``.venv`` is the preferred environment for contributors working
+on tests or documentation:
+
+.. code-block:: sh
+
+   tests/prepare.sh
+
+The preparation script installs the package in editable mode with both test and
+documentation extras:
+
+.. code-block:: text
+
+   .[test,docs]
+
+Run Tests
+---------
+
+Run the modern pytest suite:
+
+.. code-block:: sh
+
+   .venv/bin/python -m pytest --ignore=tests/legacy --ignore=tests/support tests
+
+The legacy harness remains available separately and is intentionally not part of
+the default pytest run.
+
+Build Docs
+----------
+
+Build HTML documentation:
+
+.. code-block:: sh
+
+   .venv/bin/sphinx-build -b html docs docs/_build/html
+
+The same command is wrapped by the Sphinx ``Makefile``:
+
+.. code-block:: sh
+
+   make -C docs html
+
+VS Code
+-------
+
+The repository includes ``.vscode/extensions.json`` with recommended
+extensions, including Esbonio for Sphinx language-server support. The workspace
+settings point Python tooling and Esbonio at the repo-local ``.venv``.