Bugfix/fix notebook docs

[RobbieKiwi]

Changes proposed in this Pull Request

Add highs to the requirements for building docs so that there will be a solver available

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.

[FabianHofmann]

wonderful @RobbieKiwi, thanks