Move Built-in and Custom Models to sections 3 and 4. Iris stack in section 5.
Trim file format discussion in intro. Fix 'tedious sed building' discussion
in Overview (section 2). Fix various conflicts with acronyms.
Fix #24. Moves Use Case and Custom Models to before the Iris Stack.
Also changes the title, fixes-up the intro (includes Sherpa, Specview and NED
references), and removes unecessary bullet points (not the conclusion).