create markdown for non-content files
generate index.md at chapter level

- generalize the index.md generation function so it will generate any of
  the required index.md files (course level, chapter level) based on
  parameters
add learning objectives to chapter level index.md
generate chapter reviews

- also generalize function for previously creating index.md file to
  create any type of file
generate section exercises

- also rename the main content parsing function to something more
  semantic
generate keywords.xml file for course
add sections headings to chapter level index.md