Replace plt.show() with fig.show() in the docs

[jhaigh0]

Description of work

Since upgrading to matplotlib 3.7, plt.show() has sometimes created freezing / hanging behaviour when run from within workbench. This pr replaces plt.show() uses in the documentation with fig.show() to make sure all scripts in the docs are compatible with workbench.
We'd still like to find the route cause of this problem, but there is no harm in this change to the docs which will hopefully stop as many people encountering it.

This was suggested in the comments of #36905

To test:

• cmake with docs plot option on. I could only make the docs build work on linux with this option
  • cmake -DDOCS_PLOTDIRECTIVE=ON --preset=linux
• build docs-html
• Check the files which have an uncomented fig.show() within a .. plot::
  • Sometimes the plots weren't showing up on the first build for me (irrespective of the show method), only after I made an edit to the code. Unsure what to suggest in this case ofther than putting a comment or something after the fig.show() to prompt it to re build
• Check the scripts I've changed a bit more significantly run in mantid

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[github-actions]

👋 Hi, @jhaigh0,

Conflicts have been detected against the base branch. Please rebase your branch against the base branch.