Separate Lilypond and site generation

Have the site generation as a dependency in this repo. If we have it separate, it could be possible to use an external CMS to upload files etc., making it easier to use Musecore and other software to publish.


Here is a suggested new solution:
![image](https://github.com/Snapsakademien/scores-ly/assets/1438972/7b61959b-4d97-4081-8f44-6349acacc3dc)
