Convert docs to sphinx

docs/installation.rst

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+Installation
+============
+
+FFX is designed for a Linux-like workstation with local command execution. The
+runtime media tools must be available on ``PATH``:
+
+* ``ffmpeg``
+* ``ffprobe``
+* ``cpulimit``
+
+User Bundle
+-----------
+
+The persistent user installation is prepared with the two-step flow described in
+the project README:
+
+.. code-block:: sh
+
+   bash tools/setup.sh
+   bash tools/configure_workstation.sh
+
+``tools/setup.sh`` creates the long-lived bundle virtualenv at
+``~/.local/share/ffx.venv`` and exposes the ``ffx`` command. The workstation
+script checks system tools and seeds local config directories.
+
+Local Test And Docs Environment
+-------------------------------
+
+Contributor test and documentation work uses the repo-local virtualenv:
+
+.. code-block:: sh
+
+   tests/prepare.sh
+
+The script creates ``.venv``, installs FFX in editable mode with test and docs
+extras, and verifies the Sphinx toolchain. Use check-only mode when you only
+want to inspect readiness:
+
+.. code-block:: sh
+
+   tests/prepare.sh --check
+
+Documentation Build
+-------------------
+
+After preparation, build the documentation with:
+
+.. code-block:: sh
+
+   .venv/bin/sphinx-build -b html docs docs/_build/html
+
+The generated site starts at ``docs/_build/html/index.html``.