Document API concept mapping between edx-platform and LC

[kdmccormick]

We have found many cases where there is a Learning Core concept and a parallel edx-platform concept.

AC: Write down these mappings, and explain when one should be used or the other. Where helpful for developers, add this information to the docs. Where we are missing parallel concepts, makes notes of this.

Examples:

• Do we score XBlocks, or Components?
• XBlocks types have sub-types (Problem:MC vs. Problem:DropDown). Should Components, too?

@ormsbee will fill in

[kdmccormick]

Another example: Scoring/grading a component instead of scoring/grading an xblock

[kdmccormick]

Another example: Component types and subtypes/attributes (problem and its responsetype, ORA and its sub-types, etc)

[kdmccormick]

@ormsbee could you add some more details to this?