I see the scope doesn't include writing or re-organizing new documentation. Maybe there could be some parallel initiative, though, to include the community to do this work?
For example, if a developer wants to learn how to write PHPUnit tests, what would the developer portal link them to? Manual:PHP unit testing already has a bunch of links, where the linked pages overlap in what they discuss, a lot of the examples are out of date, etc.
Anyone is free and welcome to start or support parallel initiatives - Wikimedia technical_documentation:_Friends_of_the_Docs might already be a good (and existing) place for anybody interested in improving Wikimedia documentation.
Which exact content to list in a Wikimedia Developer Portal which and which pages to link from a Wikimedia Developer Portal are one of the many things to sort out as we progress. Providing info about PHPUnit tests might be a listed item.
@KHarlan (WMF) I probably came across as way too "out of scope" in my previous comment, and need to correct myself. As part of the process we will have to make sure that the "key technical documents" linked from a future developer portal are well organized and up-to-date, and that it's clear who is responsible for each document. I have created phab:T264080 about that, to be tackled somewhere around the middle of the 2021 calendar year. Thanks again for the comment!
If anyone is interested, I'm currently working on a new guide for writing extensions. My progress for both pages (although still both WIPs) are at User:SamanthaNguyen/Guides/Writing an extension and User:SamanthaNguyen/Guides/Writing special pages. I'm currently not including parser tags/extension tags since that is now being covered by Parsoid (and I believe magic words also fall under this). Once those are closer to finished, I'll start re-expanding the pages related to content models and content handlers. Both pages assume nothing, so as to make sure it's helpful for everyone. Its meant so that those who are more experienced can simply skip sections as need be.
@SamanthaNguyen Those two links look like quite good tutorials to me (speaking as a non-developer, I understand what I'm supposed to do way better than on many of the existing pages). Thanks for working on this! I would be glad to see them moved out of the user namespace at some point! :)