code: Add a docstring for numpy, scipy packages

[jepler]

.. this fixes a problem in CircuitPython where building the docs would say "../shared-bindings/ulab/numpy/approx/index.rst: WARNING: document isn't included in any toctree"

[v923z]

@jepler Jeff, thanks for looking into this, I have slipped back with the python stubs.

I was wondering, why you were basing the PR on 2.1.5, and then I realised that that is the last release. I have been waiting for the fix in #305, but I understand that you want to have a proper anchor, so I have just generated a new tag https://github.com/v923z/micropython-ulab/releases/tag/2.6.0. Could you, please, update the PR with this? That should resolve the conflict that is caused by my having moved a function from linalg.

However, the question of the python stubs leads to another problem, namely that the documentation of the functions is scattered now. Earlier, this wasn't an issue, because the sub-modules were relatively small, so each of them had a single source file. This is no longer feasible, with numpy having close to 50 functions.

I am not quite sure what the proper solution is. We could separate the stubs into a single file, but you probably wouldn't like that. (I am no fan, either.) The other option is that the script that produces the documentation should contain the document structure. I believe the "just take all files from these directories and process them one by one" approach won't work anymore.