Add BALTRAD shortcourse notebooks to repo

[kmuehlbauer]

It seems a bit counter-intuitive if we add the BALTRAD notebooks during the docker build process.

I'd suggest to add the notebooks directly to the repo and link them in the TOC.

Let's add them one by one, after #57 is merged.

• https://github.com/DanielMichelson/baltrad_short_course.git
• https://github.com/DanielMichelson/baltrad2wradlib.git
• https://github.com/DanielMichelson/pyart2baltrad.git

[kmuehlbauer]

@mgrover1 Would it be possible to add the data used in these notebooks to the S3 bucket, too?

[kmuehlbauer]

@DanielMichelson when #69 is merged, all BALTRAD notebooks are supposed to run properly in CI and on the pythia hub. 🚀