Spring Cleanup: Deps and Markdown guide

[seshrs]

This PR contains annual dependency updates and a docs update. Most significantly:

• Clean up gemspec. (Primer Spec is not distributed as a Ruby gem.)
• Clean up Gemfile. (Apparently, the github-pages gem includes all the dependencies we need, so I’m updating the docs to recommend that.)
• Update dependency versions (and accommodate a webpack API change)
• Write a new markdown guide. I’ve come across obscure features like abbreviations, task lists, footnotes and pycon syntax highlighting, so I thought I should include them in a guide.

[github-actions]

The spec from this PR is available at https://preview.sesh.rs/previews/eecs485staff/primer-spec/153/.

^{(Available until Fri Mar 18 2022.)}