docs: add subplot-mosaic string compact notation

[dmatos2012]

PR Summary

This PR would close #22917 which adds a small paragraph to the docs that shows the string compact notation for suplot_mosaic. Additionally, I took the liberty to fix a typo and add a section title to the current tutorial that I believe improves readability a bit.

PR Checklist

Tests and Styling

• [ N/A] Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• [N/A] New features are documented, with examples if plot related.
• [N/A] New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• [ N/A] API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).
• Documentation is sphinx and numpydoc compliant (the docs should build without error).

[oscargus]

Looks good! Some minor suggestions, so I will not merge it. If someone disagrees with my suggestions, feel free to merge.

[timhoffm]

Thanks @dmatos2012!