update docs for consistency

adjust the index file to be more readable for the field guide

adjust titles to make more sense when displayed in top level index

change theme to 'nature', which is closer to the openstack default
style. Will eventually write our own style here.

Change-Id: I4f5fed65f9e26a4e4d333b1be4ed02c9ec246b9a
Merge "update docs for consistency"