doc: Add one of our simplest examples (hello)

Signed-off-by: George K. Thiruvathukal <gkt@cs.luc.edu>
examples: Add a few comments to hello.c to allow for Sphinx code snippets

Signed-off-by: George K. Thiruvathukal <gkt@cs.luc.edu>
doc: Add more text for introductory example (hello.c)

Signed-off-by: George K. Thiruvathukal <gkt@cs.luc.edu>