Improve tests about directory HOT 5 OPEN

@pathawks Before we make this the official home of all plugins & themes, let's come up with tests to enforce all the correct fields are present, etc.

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I'd like to create a test that makes sure anything claiming to be a Gem is actually listed on RubyGems.org

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Currently, CI will verify that no URLs 404. Should be not so difficult to do something similar for Gems.

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We should exclude urls from https://github.com/jekyll/plugins/edit/master from HTML-Proofer, as pages that have not yet been added will not yet exist.

We should type check all the YAML of themes and plugins.

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This issue has been automatically marked as stale because it has not been commented on for at least two months.

The resources of the Jekyll team are limited, and so we are asking for your help.

If this is a bug and you can still reproduce this error on the master branch, please reply with all of the information you have about it in order to keep the issue open.

If this is a feature request, please consider whether it can be accomplished in another way. If it cannot, please elaborate on why it is core to this project and why you feel more than 80% of users would find this beneficial.

This issue will automatically be closed in two months if no further activity occurs. Thank you for all your contributions.

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