Minor updates to Jupyter Book (badges, typos, clarity)

[shawnrhoads]

This PR updates badges and years, and incorporates the following minor recommendations from the JOSE reviews:

A couple of typos in the Modules section of "Intro to Python Basics": "python import module_name", "python import module_name as new_module_name".

• fixed typo

I wasn't sure why some notebooks show the cell outputs (e.g., "Working with Data", "Linear Modeling") while others do not (e.g., "Intro to Python Basics", "Nonlinear Modeling) - in the built site, that is.

• removed cache for jupyter book build

You might not want to add in any extra syntax confusion, but the extraneous cell output that gets produced sometimes by matplotlib (e.g. <matplotlib.legend.Legend at 0x174e572d908>) can be suppressed by adding a semi-colon to the end of the command (e.g., plt.legend();) - in case you weren't aware of that.

• added in docs/module-01-02_Working-with-Data.ipynb

The colour scale for the heatmap of the "Least-squares minimization" section makes it difficult to see the minimum (on my screen, anyway) - there are many cells that are black.

• changed the colormap and also added a rectangle around the minimum