Add National Gallery of Denmark as data source

## Problem
National Gallery of Denmark (SMK) ([API documentation](https://www.smk.dk/en/article/smk-api/)): The National Gallery of Denmark (SMK) is Denmark’s largest art museum, home to a vast collection of Danish and international art spanning seven centuries. Its API provides access to digital records of artworks, artists, and exhibitions. It has a collection of about 260,000 artworks. Information about license types can be found here: [Copyrights](https://www.smk.dk/en/article/rights/)

## API Information:
- No API key required
- Data available through JSON format
- No query limit

## License/copyrights information ([Copyrights](https://www.smk.dk/en/article/rights/))
A large part of the art collection is in the Public Domain and part of the shared cultural heritage which due to age is free of copyright. Some artworks are under copyright when the artist is contemporary or passed away less than 70 years ago. Images on the website that are not in the Public Domain or protected by copyright, are released under the Creative Commons-license CC BY-SA 4.0 (Attribution-ShareAlike).