Updating user guide docs to add more info/edits

Created by: cmtool

More fleshing out of the user guide, with particular emphasis on adding links to CiviCRM.org resources, such as the Case Studies, Statistics page, and other things aimed at smoothing the process of learning about CiviCRM. Additionally, when discovered, removing references to no-longer-existing resources like the forums, book.civicrm.org, etc.