Add Literature: Contracts-for-difference to support renewable energy technologies

[KrainerD]

https://op.europa.eu/en/publication-detail/-/publication/0cba0fa5-f2fa-11ee-8e14-01aa75ed71a1/language-en

Add EU publication: Contracts-for-difference to support renewable energy technologies (Considerations for design and implementation)
Overview of CfD Design, Discussion and country examples (Kietzing)