Include standard page layout

1. Add `_layouts/page.html` (which was accidentally being excluded by an overly-zealous `.gitignore`).
2. Modify `.gitignore` so that it doesn't exclude HTML files.

Note: when the build is being tested in this repo, it creates HTML files in the root directory that should *not* be put under version control.  When it is cloned as a starting point for an actual lesson, though, the HTML files in the root directory *should* be committed, so the safest thing is to modify `.gitignore` so as not to ignore them, and trust that whoever's working in this repo doesn't accidentally add unwanted HTML files.

Closes #5.