Adding previous/next links at the bottom of episode pages.

1.  Put prev/next finding code into separate include.
2.  Modify episode_title to use that.
3.  Include it in the footer and add prev/next arrows if there's a match.

Note: the prev/next finding code must be added to *both* episode_title and footer because variables set in includes aren't exported to other includes.

03-organization: Motivate AUTHORS by appealing to convenience

Fixing macro-generated URLs to remove double slashes.

Switching from .getready to .checklist

CONTRIBUTING: Split development notes into their own file

This closes #1.

03-organization: Drop _extras/contributing.md reference

02-formatting: Fix "lerners's" -> "learners'" typo

01-tooling: Fix 'jeyll-collection' -> 'jekyll-collection' typo

01-tooling: Drop remotes from fork/clone discussion

reference: Add a "Glossary" header

index: Add trailing period for "Every lesson's ..." sentence

CITATION: Fix 'new-lesson-tepmlate' -> 'new-lesson-example'

The file is not *needed* for anything, because the information is in
the Git history.  It's just easier to access when it's committed to a
file in the repo than it is to ask curious folks to clone the repo and
run ‘git shortlog’, etc.

This file was removed in 2e05819f (Naming layouts and inclusions more
clearly, 2016-04-19).