Add diagram of episode layout with notes

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