Update docs index page
Update docs index page

- Switch from proposing Ubuntu 16.04 to 18.04 as the most tested
  platform.
- Make it clearer that creating an additional "stack" user is
  optional when running on a cloud image, as this step often leads
  to errors for new users.
- Fix some minor nits along the way.

Change-Id: I39aef1a230b668b932b1681fcd0deeb423b411f1

nawawi/openstack,dtroyer/devstack,openstack-dev/devstack,dtroyer/devstack,nawawi/openstack,openstack-dev/devstack