MAINT: create `[dev]` extras short that installs all optional and test and docs build dependencies

[bsipocz]

Genai is pretty bad giving advise about dependecies, and apparently it misses the [docs] and [test] options from the landing page. So maybe we need to create a new umbrella [dev] that includes everything and the kitchen sink as a short for developers. (as [all] is for end users and therefore doesn't include test and docs dependencies).

And also, while we're at it, the testing.html page is seriously outdated, and at bare min should include some hint about what packages needed for testing.