Render wradlib-notebooks as own document at readthedocs

[kmuehlbauer]

Currently the notebooks are rendered via GitHub Actions and then pulled in by wradlib-docs to be rendered in the wradlib documentation.

This workflow is inherently error prone. To get this workflow more maintainer friendly the idea is:

1. DOC: reintroduce doc into wradlib repository wradlib#619 over at wradlib
2. a. render the notebooks on push to wradlib and push them to devel-branch (or tag-branch) here depending on wradlib version commit
   b. render notebooks here and push devel-branch (or tag-branch)
3. render new notebook-documentation on rtd with the just rendered branch
4. a. link the notebook-docs as subproject to wradlib documentation from the above rtd
   b. render the notebooks within new wradlib doc rtd processing after (2.a)

This might take a while to set up and test thoroughly. For the time being the automatic rtd builds are disabled here #78 and on wradlib.

[kmuehlbauer]

It looks like with wradlib/wradlib#619 we have everything in place.

The notebooks are rendered either on push to wradlib or here on push to main. ReadTheDocs is triggered and the documentation is build as usual. That's for now the minimal-invasive action.