FC-0068 docs: Update How-tos edunext 1

[Apgomeznext]

This PR cleans the How to Configure Your Course for Content Experiments and Add Content Experiments to Your Course, creates the new documents necessary to accomplish the Style Guide and the Diataxis Criteria, and tests the Maintenance Chart in some of these documents.

[openedx-webhooks]

Thanks for the pull request, @Apgomeznext!

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[sarina]

Why delete this note, and the L22? It might not be obvious how the phrase split_test relates to experimnets, and L22 tells in advance what the steps will walk you through

[Apgomeznext]

This information is passed to the specific How-to create_content_experiment.rst (source/educators/how-tos/advanced_features/create_content_experiment.rst). I had to break this how-to into several documents, but I can add that information here again if necessary.