make 'pip' an independent section

Previously it was (incorrectly) a subsection of the 'conda' section.

content/install.rst

@@ -256,7 +256,7 @@ channel (only availible for GNU/Linux)::
 
 
 ``pip``
--------
+=======
 .. caution::
 
     Installing Kwant with ``pip`` is not easy because Kwant has several
@@ -287,7 +287,7 @@ and see `pip documentation <https://pip.readthedocs.org/>`_ for a detailed
 description of ``pip``.
 
 Installing non-Python dependencies
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+----------------------------------
 
 As mentioned above, ``pip`` will not install any non-Python dependencies
 required by Kwant. Kwant has several non-Python dependencies: