When --enable-editable is in use, the Jupyter kernel sagemath is not being installed.

Workaround for users:

• Either install without --enable-editable first
• Or fix it using: ./sage -python -c 'from sage.repl.ipython_kernel.install import SageKernelSpec; SageKernelSpec.update()'

In this ticket, we fix it by adding a custom develop command.

(In follow up ticket #30298, the jupyter kernel (but without the symlink doc -- which cannot and should not be stored in a wheel!) will also be added as data_files - and thus be available in all types of installations - setup.py install, wheel-based, and editable)

CC: @tobiasdiez @dimpase @kiwifb @antonio-rojas @egourgoulhon

Component: build

Author: Matthias Koeppe

Branch/Commit: 4473375

Reviewer: Tobias Diez

Issue created by migration from https://trac.sagemath.org/ticket/33855