Improving Mistral example

[jugi92]

• using python to download example file because wget is not available on all os types
• including output to show it worked
• giving an example for the foundry endpoint as it might be confusing which one to use
• checking if the correct file is read, as users would change the path and might not point to a file. Previously the notebook would still run and create throw an internal server error. The assertion makes sure there is something in the document.