This PR adds a new notebook that shows how negative electricity prices can be reproduced with a linearized unit commitment model. Such prices appear in real markets when generators with start-up costs and minimum generation limits find it more economical to offer electricity at a negative price (effectively paying to stay online) rather than shutting down and restarting later.

For example, such a situation occurred in Germany on Week 15 of 2025

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in docs.
• A note for the release notes docs/release-notes.md of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.