Some minor clean up of various doc pages

* simplify the links on index.rst
* update the main description on the index page

* move install/ironic-install under deploy/
* rename ironic-install to install-guide
* rename userguide to user-guide
* clarify a few things in the install-guide

Change-Id: Ic6eff035df42d0c3fa78f54eac7ad8322ebb0afc
Merge "Some minor clean up of various doc pages"