Fixes #546 by using level-3 headings in all Markdown source files for subsections.
We continue to use level-4 headings for objectives, key points, callout boxes, and challenges.
Fix more level 3 headings
Merge pull request #548 from gvwilson/using-level-3-headings-for-subsections-in-markdown

Using level-3 headings in Markdown files for subsections.