Added Product Overview, Login, Quick Reference

- Added the product overview section.
- Added the login screen mockup and the (related) forgotten password screen mockup.
- Added the quick reference section. Note all the bolded words in the document are
  screens. Most of these screens are ones that will be added to the document, but
  since they aren't already, I haven't added a page reference to them. When you add
  them, please make a label and add a page reference to the screen in the Quick
  Reference section. If you make a modification to the way that your section works
  that differs from the way it is laid out in the Quick Reference section, please
  either change it, or let me know so I can do so. There are also a few screens that
  seem necessary to me in the Quick Reference that seemingly don't have any place
  in the rest of the document, and I'm unsure what to do about those.