Add SVG diagrams of core pandas operations (merge/join, groupby, reshape) to pandas.md

This is a nice reference point for when we review the pandas lecture

https://betterprogramming.pub/pandas-illustrated-the-definitive-visual-guide-to-pandas-c31fa921a43

It contains many useful visualisations of various pandas operations that make learning pandas really simple.

Our own datascience series has some of these features (but they are cumbersome to build, @sglyon and @blkboxeconomist are geniuses when it comes to writing raw svg animations 😄). The link includes a lot of really interesting visualisations of pandas operations.

Ideas:

1. see if there is a pandas visualisation app that builds these based on an operation?
2. use AI to build visualisations to show how they work in our lectures
3. Link to resources like this?