Communities should be aware that MediaWiki:NewcomerTasks.json file can have effect on their newcomers, and should be informed on how to edit it. This relates to Growth-Scaling a bit, but it's an improvement rather than a blocker, so not tagging that tag.

This is filled as a follow-up to this comment by @Tgr:

In T266020#6807888, @Tgr wrote:

The documentation says they should not have a namespace. The way the code is written now it would work both ways, but maybe it's better not to assume that.

We should add a message editnotice-8-NewcomerTasks.json to inform communities about how they can maintain the file, linking to docs.

@Tgr: Where are the docs, please? I can't find anything at mediawiki.org.