Add entry for "derating factors" to Wissen

Like other "Begriffsdefinitionen", e.g., [Kapazitätsmärkte](https://en.ergie.at/wissen/kapazit%C3%A4tsm%C3%A4rkte/)

<img width="910" height="110" alt="Image" src="https://github.com/user-attachments/assets/35fd1f9a-ee51-4cbd-9cb6-19b33550dd56" />
Source: ??? (from D.K.)