Adding link to syllabus (episodes) from setup instructions

Mention that R packages are needed to build example lesson.

1.  Adding link using `page.root`.
2.  Adding ID attribute to syllabus.

Replaces #56.

fixing name of lesson_initialize.py script in point 11

Disambiguating "guides"

Adding a footnote to the schedule that it may vary.

Fix "br" typo causing ugly rendering (epub only)

</br> instead of <br/> renders fine in the browser but not in an epub

1. Renaming episode_title to episode_navbar since it now has dual use.
2. Removing hr elements that look odd with footer nav arrows.
3. Conditionalizing display of episode title.
4. Modifying layouts for breaks and episodes to include the footer nav arrows.

Closes #35.

1.  Fixing image link in `setup.md`.
2.  Regenerating all-figures.

Closes #42.

An internally-defined Markdown link has the form [xxx][yyy].  If the
ID 'yyy' doesn't resolve, the text is left as-is, so we check for
that, then subtract those that reference configuration values using
'{{'.  It's a hack, but it'll catch at least a few things.