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. 
This is because the document is parsed as XML, so if you have invalid embedded HTML, it simply won't parse.