Update URLs in RAG demo

[helena-intel]

The existing URLs now resulted in empty documents and a cryptic error. With the URLs from this PR the notebook works as expected.

Also added ipywidgets to requirements.txt (to prevent a message about it) and changed !pip to %pip because it should be safer with virtualenvs.

FWIW: I also had to build OVMS from source to run this demo, it didn't work with the image pulled from Docker hub