Add Collection Types doc

[verythorough]

Collections are explained briefly in the Quick Start, and nowhere else. I've included most configuration options in the proposed outline for a Configuration doc (#685), but the collection types of file, folder, and filter are complex enough to require their own doc. Here's a rough outline pulled from #598 (with the order changed slightly):

Collections: file, folder, filter
(explain the differences, show config examples for each)

• folder:
  • grouping of separate files based on the same template, stored in the same folder
  • UI widgets will be identical for all items in the folder (or all matching the filter, below)
  • may or may not be able to add new items (depending on create field)
• filter:
  • used with folder type
  • can be used to pull only items matching the specified field value
  • requires field and value: must be an exact match
• file:
  • single file or grouping of files, with fields specified for each file individually
  • can be useful for data files with settings or highly customized pages
  • cannot be filtered; cannot add new items
• Note: regardless of collection type, each item in the main area of the collection browser represents a separate file (though separate files may be combined into a single published page via templating)

[verythorough]

I forgot to mention that @tech4him1 started some work in this direction with #656 (then closed it in favor of this outline). Much of the work he did there pulls from the Quick Start, but I think we'll probably want to be more "reference-like" and less "instructional" with the structure of this doc, if that makes sense.