Add how-to change plotting backends guide

[r3kste]

This PR adds documentation on using different plotting backends.

Note: This PR should go after #641, as this relies on some fixes made to criterion_plot()

Summary of Changes

1. Created how_to_change_plotting_backend.ipynb with information on:

   • Supported backends
   • Type of the returned figure object
   • Simple usage example for criterion_plot() for each backend.

2. Updated how_to_visualize_histories.ipynb with a small section on changing backends.

To-do

• Add fix for matplotlib not rendering the figure upon re-running cell.
• Should we document 'advanced' usage (customizing plots after generated by optimagic)?

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[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/optimagic/visualization/backends.py	75.00%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (75.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Files with missing lines	Coverage Δ
src/optimagic/visualization/backends.py	97.67% <75.00%> (-2.33%)	⬇️

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[r3kste]

The failing test is unrelated to the changes.

Resolved after re-running the test.

[r3kste]

Codecov is failing because plotting is not being tested in an interactive environment. I could try and add a simple smoke test for test coverage. Thoughts? @timmens

[timmens]

No, if the PR is ready, I'll try to merge it without the checks passing. Makes no sense in this case. Thank you!

Is it ready for review? @r3kste

[r3kste]

Yes. It is ready for review @timmens

[timmens]

Very nice! I have two relatively small remarks. Once these are done, this is ready to be merged! Thanks a lot.