benchmark

Fix links to further doc in user_guide.md (#1215)

Refactoring in 201b981a moved most of the documentation from `README.md` to `docs/user_guide.md`. Some links from `README.md` to other `docs/*.md` files ended up unchanged in `docs/user_guide.md`. Those links were now broken as they did not link from outside the `docs` directory anymore, but from inside it. Removing the leading `docs/` for these links fixes this.