Documentation: Clarify blueprint extends, how it works and what is allowed and what not

[renestalder]

It's possible to reuse the status definitions (draft/listed etc) and that might be something everybody will ask themselves very early on. My experience with beginning with Kirby was exactly this: Create blueprints after blueprints and then wonder, how I can reuse frequently used definitions, and in this case, the status definition was the most obvious one I wanted to reuse.

I only looked this up today and found the answer by texnixe in the forum.
https://forum.getkirby.com/t/extends-and-reuse-for-status/17192/4

I believe it would make sense to extend the part of the documentation of "extending blueprints" with all possibilities that we have in extending from other files and also sometimes link back to this page from other parts of the documentation, e.g. when on the Page Blueprints documentation looking at "Statuses", mentioning how to reuse statuses for multiple pages.

Until now, it's not entirely clear to me how Kirby's extends work. Other tools also using YAML, also implement a form of extends, e.g. GitLab CI, but all those extends work differently, like some use relative paths to another YAML file including the file extension, others like Kirby have some form of namespace handling starting from the site director, e.g. fields/my-fields. For quite some time I thought there is a predefined way of how the blueprints in the site folders have to be for this to work. But now I believe, it just picks up whatever folder names you choose, e.g. extends: options/status. However, I didn't find this to be explained to me.