Doc 224

@lamagnifica @mhoeber This is the reorg of the problems (it's a giant PR so it may take a while to review):

I created a "Working with Problem Components" file in the "Creating Content" folder. The WwPC file has general information about problem components--what problems look like to students and an explanation of the UI in Studio, as well as info about problem settings and a couple of other things common to all problems.

I created an "exercises_tools" folder. This folder contains a "Creating Exercises and Tools" file that lists all of the exercises and tools (grouped as "General," "Image-Based," "Multiple Choice," and "STEM"), as well as one file for every kind of exercise and tool. 

I moved all the content from the former Appendix E into the files for the individual exercise/tool types.

I created a "Math Response Formatting for Students" file and put it in the "students" folder. This file lists text that students can enter when they answer math problems.

I updated the formatting cheat sheet headings to be consistent with the headings in the rest of the Studio guide and added sample table formatting as well.