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'

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.