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).

GitHub repositories know the repository they were forked from (sort of
like the usual 'origin' remote), but I'm not aware of them having
other remote-like references.

Besides dropping the "other remotes" bit, soften the 'origin' wording.
I'm not sure if GitHub uses 'origin' remotes or not to track the
parent repository, and the implementation details matter less than the
fact that there *is* a reference of some sort to the parent.

To separate that content from the "Key Points" injected via
_layouts/reference.html.

The previous README contained notes for example-lesson developers
(e.g. Git workflow notes) as well as notes for lesson consumers
(e.g. _config_template.yml copying).  This commit splits the former
into GitHub's preferred file [1].  That ensures the license
information, code of conduct, and other development-critical notes are
highlighted and easily accessible to anyone filing and issue or
pull-request against this repository.

[1]: https://github.com/blog/1184-contributing-guidelines

Earlier version was matching episode.title with page.title to find the episode object for a page when generating next/prev links.
This fails when several episodes have the same title (e.g., "Morning Coffee" for a two-day workshop schedule).
This fix matches episode.url with page.url instead.

This fixes https://github.com/swcarpentry/instructor-training/issues/160.