docs: Actually add unix port indexes, so docs for it could be generated.

docs/unix_contents.rst

+MicroPython documentation contents
+==================================
+
+.. toctree::
+
+    library/index.rst
+    reference/index.rst
+    license.rst

docs/unix_index.rst

+MicroPython documentation and references
+========================================
+
+.. toctree::
+
+    library/index.rst
+    license.rst
+    unix_contents.rst
+
+Indices and tables
+==================
+
+* :ref:`genindex`
+* :ref:`modindex`
+* :ref:`search`