Such things already exist in piecemeal (like the link I shared), but having a structured course would be a welcome improvement.
Further, a good series of YouTube videos or web course with the intent not to get you to advanced JavaScript programming (as this is retreading work done elsewhere), but to get a beginner familiar enough to use it and provide a practical use overview of how to use the API for performing common (and even intermediate level) tasks would certainly be a great project for a community member to provide.
Having said that, I strongly agree that the API documentation could benefit from more examples, but Atropos has mentioned that the new documentation for the upcoming 0.8.x release will include vastly more examples (if not an example for every API method/property). Once you have learned JavaScript (all the way up through React, jQuery, CSS and so on if you choose, though they are overkill for simple macros, though perhaps not for module or system dev), then you can use the documentation that Foundry provides in order to access the API that it provides so that you can use your existing programming skills in order to interface with its capabilities, data and so on. Foundry doesn't have to provide a course or guide to teach it to you because that is already done more than adequately in any number of online courses, in-person courses, and web guides. This makes your comparison with Roll20 somewhat unfair - the "language" for PowerCards is non-standard - it is wholly a Roll20 construction and thus that is the only place you could learn or use it.
It isn't a course to teach you JavaScript as there are plenty of those all over the web. The link I included in my post does have well documented macros to use as a basis for creating your own.