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

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And also, while we're at it, the testing.html page is seriously outdated, and at bare min should include some hint about what packages needed for testing.

Start by locating the dependency-extra definitions for [all], [docs], and [test], then read testing.html and compare it with the current testing requirements. Done means the [dev] extra covers the requested optional, test, and documentation build dependencies, and testing.html gives developers a current indication of the packages needed for testing.

Written by the indexing model from the issue text.