Following #25198, which removed the discouraged form constrained_layout=True from examples, this PR does the same for the tutorials and the user guide. Also put the phrase "constrained layout" in italics consistent with #25144.

Although this is just documentation, I milestoned this for v3.8: it partially builds on #25144 which was not backported due to conflicts, so conflicts for this one will likely be worse!

PR Summary

PR Checklist

Documentation and Tests

• Has pytest style unit tests (and pytest passes)
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• New plotting related features are documented with examples.

Release Notes

• New features are marked with a .. versionadded:: directive in the docstring and documented in doc/users/next_whats_new/
• API changes are marked with a .. versionchanged:: directive in the docstring and documented in doc/api/next_api_changes/
• Release notes conform with instructions in next_whats_new/README.rst or next_api_changes/README.rst