Add qu2cu to docs structure.

Doc/source/qu2cu/cli.rst

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+###################################################
+cli: Quadratic-to-cubic converter command-line tool
+###################################################
+
+.. rubric:: Overview:
+   :heading-level: 2
+
+.. automodule:: fontTools.qu2cu.cli
+   :members:
+   :undoc-members:
+     
+    .. rubric:: Module members:
+       :heading-level: 2

Doc/source/qu2cu/index.rst

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+########################################
+qu2cu: Convert quadratic curves to cubic
+########################################
+
+.. rubric:: Overview
+   :heading-level: 2
+
+
+Routines for converting quadratic curves to cubic splines, suitable for use
+in TrueType to CFF-flavored OpenType outline conversion.
+
+The basic curve conversion routines are implemented in the
+:mod:`fontTools.qu2cu.qu2cu` module.
+
+.. note:: The redundancy in the module name is a workaround made
+	  made necessary by :mod:`fontTools.qu2cu`'s usage of
+	  `Cython <https://cython.org/>`_. Providing Cython support
+	  for the module enables faster execution on systems where
+	  Cython is available. However, the module remains fully
+	  available on systems without Cython, too.
+
+.. automodule:: fontTools.qu2cu.qu2cu
+   :members:
+   :undoc-members:
+
+    qu2cu also includes a submodule that implements the
+    ``fonttools qu2cu`` command for converting a UFO format font with
+    quadratic curves into one with cubic curves:
+		   
+     .. toctree::
+        :maxdepth: 1
+
+        cli
+
+    
+     .. rubric:: Package contents
+        :heading-level: 2
+