Add Missing Data notebook to Polars course

[etrotta]

📝 Summary

Add a Notebook to the polars course ( #40 ) covering common methods to deal with missing data, relevant polars methods and includes an example dealing with real world data.

Supersedes #96

📋 Checklist

• I have included package dependencies in the notebook file using --sandbox
• If adding a course, include a README.md
• Keep language direct and simple.

[etrotta]

At first I was planning to work on top of folick's branch, but figured it was better to rewrite considering I felt a need to change the dataset, which implies changing most of the code

I still took a bunch of inspiration from it though, so probably should credit it in some way? Not sure how exactly though, should I just change the credits bit from

-    _by [etrotta](https://github.com/etrotta)_
+    _by [etrotta](https://github.com/etrotta) and [Felix Najera](https://github.com/folicks)_
# or maybe
+    _by [Felix Najera](https://github.com/folicks) and [etrotta](https://github.com/etrotta)_

or something else

[Haleshot]

should I just change the credits bit from

-    _by [etrotta](https://github.com/etrotta)_
+    _by [etrotta](https://github.com/etrotta) and [Felix Najera](https://github.com/folicks)_
# or maybe
+    _by [Felix Najera](https://github.com/folicks) and [etrotta](https://github.com/etrotta)_

This works!

[Haleshot]

Really nice flow throughout; the mini case study + relevant maps/viz go well with the notebook topic at hand. Merging! @folicks, let me know what you think of this notebook as an improv: over #96

Also, nice job on following the notebook templates made so far listing the Utilities/appendix section at the end; reminds me of Vincent’s/marimo's recent demo (video). Wonder if we can also get a column-like layout going.