Fix broken links on install/index.rst

Currently the links in the section summary on the index.rst/index.html
file are broken. Can be reproduced by going to
http://ipython.readthedocs.io/en/stable/install/index.html and clicking
on *installing IPython itself*, or *kernels for Jupyter* links.

Specific changes
----------------
* Added a reference label to the `install.rst` file
* Modified links from external link format to Sphinx arbitrary location
cross-referencing format in the `index.rst` file

Testing
-------
Changes have been tested with a local sphinx build through the supplied
makefile and specific links touched are fixed. Tested a few possibly
overlapping links (Jupyter:install for instance) and they seem to be
unaffected by the change.

docs/source/install/index.rst

@@ -14,8 +14,8 b' Installation'
 
 
 
-This sections will guide you through `installing IPython itself <install>`_, and
-installing `kernels for Jupyter <kernel_install>`_ if you wish to work with
+This sections will guide you through :ref:`installing IPython itself <install>`, and
+installing :ref:`kernels for Jupyter <kernel_install>` if you wish to work with
 multiple version of Python, or multiple environments.
 
 

docs/source/install/install.rst

@@ -1,3 +1,5 b''
+.. _install:
+
 Installing IPython
 ==================