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.