Add information for instructors to parts & chapters

[rzach]

Add information for instructors/remixers to parts, chapters and if appropriate even individual sections to (a) warn about shortcomings ("this section needs more explanatory text"), (b) highlight relation to other parts ("This chapter assumes the reader is familiar with Turing Machines/relies on the part turing-machines), (c) alert instructor to configuration options (esp tags) that are relevant ("There are two chapters on proof systems, use the prfND and prfSC tags to include or exclude material on natural deduction and the sequent calculus").