The MuseScore community maintains collaboratively written handbooks and a collection of help pages. Any logged in user can edit and improve the pages of the handbook. If you see a mistake or something missing in the handbook, don't hesitate to fix it or extend it—click the ⠇ to the right of the title, choose "Edit", and improve the page. For more information about how to add pages etc., see the Administrative guidelines. To create a "How to" and Tutorials with screen capture, you can record a video, upload to YouTube and post a link, alternatively use the free LICEcap to generate and post GIF.
Handbooks are written in English and translated into other languages. You can help us with translations.
Developers of main app use the wiki on github.com now, you need to login to your github account to edit it. The old developers' handbook here does not require github account, it still contains some useful info.
Developers of plugins use the plugin section in developers' handbook
Developers of MuseScore 3.x compatible soundfont use the soundfont section in developers' handbook
In cases where you would like to discuss changes, post in the documentation forum.