modular/shell: Start reorganizing the shell material

modular/shell: Start reorganizing the shell material

Mostly split the examples out into their own directories with READMEs,
but there was a lot of editing to partially convert the long-form
notes into a condensed instructor.md targeted at instructors.  I've
gotten to the pipes section and run out of steam.  Erik may be taking
over the rest of the rewrite...

W. Trevor King: I dropped everything from the original 7ba5e9d except
for the modular/shell/exercises/hearing/README.md addition.  I also
merged in the history of the shell notes from
git://tremily.us/swc-modular-shell master so you can follow the
development of the README content.