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. 
nit: 'common' is inaccurate.