Adding missing figures in demos.

[gvegayon]

This is a minor fix (but important). It seems that Quarto has issues converting files to RST format. Python qmd figures should have the following:

1. Explicitly plot the figure using plot.show().
2. Have the code-chunk label with no empty spaces and starting with the fig prefix.
3. Have a caption.

Here is an example:

#| label: fig-plot-hospital-admissions
#| fig-cap: Daily hospital admissions from the simulated data
...
plt.show()

If either of these three is not fulfilled, Quarto+sphinx don't process the document correctly.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.01%. Comparing base (d12d2d1) to head (e82e55b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #110   +/-   ##
=======================================
  Coverage   92.01%   92.01%           
=======================================
  Files          34       34           
  Lines         664      664           
=======================================
  Hits          611      611           
  Misses         53       53           

Flag	Coverage Δ
unittests	92.01% <ø> (ø)

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