Multiple changes based on converting Make lesson to template

Converted the existing lesson Make to the new template - result is in https://github.com/gvwilson/make.  Learned a few things from doing that, so updated the lesson template:

1. Removed requirement for keywords in index page.  (We'll wait until we actually know how we want to do this before requiring it.)

2. Added a discussion page for advanced notes, pointers to other material, etc.

3. Settled on a single page layout `_layouts/page.html` with `title` and `subtitle` fields for all pages other than the slides.

4. Added a target to the Makefile to delete generated HTML (@aaren was right...).

5. Rewrote parts of `README.md` to describe these changes.

6. Settled on using **bold face** to mark glossary entries.

7. Used four-space indent everywhere in our Markdown.
What about move this variable to Makefile?