Merge pull request #747 in SNORT/snort3 from doc_umentation to master

Merge pull request #747 in SNORT/snort3 from doc_umentation to master

Squashed commit of the following:

commit 1871bd514dce2bfaa7d28ccf4df25d2d5f48b00d
Author: Russ Combs <rucombs@cisco.com>
Date:   Tue Dec 13 07:55:26 2016 -0500

    more reorganization for clarity

commit 6db3bc35d92ae7ccc2bf9079d94b2aa5c5c7ae2d
Author: Russ Combs <rucombs@cisco.com>
Date:   Tue Dec 13 06:17:12 2016 -0500

    use Snort 2 and Snort 3 throughout

commit ea75de13ba40bd1acb479c5380fb898d024ab997
Author: Russ Combs <rucombs@cisco.com>
Date:   Fri Dec 9 11:45:49 2016 -0500

    doc: refactor user manual to start with overview, getting started,
    concepts, and features sections.