add cover for sidenvägen

[Sixten]

https://runeberg.org/siden/0001.html

The source says this: "Project Runeberg (runeberg.org) is a volunteer effort to create free electronic editions of classic Nordic (Scandinavian) literature and make them openly available over the Internet."

Here is more info if needed: https://runeberg.org/admin/

If the sight is not accepted you can use this image instead, it is from the Swedish national library: https://libris.kb.se/bib/7152541

[Martin]

Cover added from library source.