First pass at improving OLX docs

@mhoeber @cpennington This is a first pass at fixing some of the issues with OLX documentation. Changes: 

* Correctly documents OLX structure, both with regards to course hierarchy and with regards to how directories are structured. Does not assume either a Studio layout or an all-in-one-file layout. Does not assume children have to have fixed format. 
* Provides some minimal high-level documentation on the structure of OLX. 
* Breaks up components between those which go in problem (inputtypes, responsetypes, etc.) and those which go in the top-level XML (XBlocks in course.xml). Right now, these were mixed. 
* I don't assume the only possible types of XBlocks are HTML, Problem, and Video. 

Potential issues with PR: 

* I do not know RST very well. I did not confirm all docs were valid .rst. I edited in emacs. It would be good to confirm this. 
* I did this fairly quickly. Copy-edit would be very helpful. I'm sure there are issues. 

Unresolved in this PR: 

* We need much more high level documentation about the overall structure of OLX. This jumps into details before giving a framework for thinking about OLX. I believe myself, Cale, Ned, and Dave are the people who could write or explain this. 
* We need clear examples. 
* Youtube documentation is confusing. Studio exports include redundant information. We need to figure out which of the two is required. 
That's correct. The phrasing here could be better, but there is an important distinction to be made. 

From the point of view of our audience: We're trying to get this as a standard. The course hierarchy is generic. It could run in Moodle, Sakai, Canvas, etc. if they were to adopt OLX. MOOC advertising pages, policies, etc. are edX-specific. Those are unlikely to be part of the standard except as having a mechanism for platform-specific extensions and metadata. 

From the point of view of course authors even limited to just edX: There are three distinct parts of OLX/edXML: 

* Things in JSON, as well as other ad-hoc formats. 
* Things in top-level OLX. 
* Things in (capa-style) problem XML. 

Making this distinction is important.