Update to jekyll 4.0.0

Dennis Leerequested to merge
dennisleexyz/jekyll:master into master

This branch brings the jekyll version from 3.4.0 to 4.0.0, while preserving the changes specifically for the GitLab Pages examples.

Preview the output at https://dennisleexyz.gitlab.io/jekyll/

Also encompasses the changes described in !40 (closed).

Edited by Dennis Lee

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