FC-0068 docs: Reference cleaning by edunext #7

[Apgomeznext]

This PR modifies a series of reference documents migrated from the legacy repositories to the educators' documentation.

The principal changes are in the add_link_website_unit_file, which was modified to be a how-to and create two how-tos based on testing_course_content.

In this PR, some articles maintain links that direct to edX legacy repositories, as the content has not been migrated to Open edX repositories.

[openedx-webhooks]

Thanks for the pull request, @Apgomeznext!

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