Docs: notebook download links to current notebook version

[merlinND]

There are two downsides with the current notebook download buttons:

• Because they point to https://raw.githubusercontent.com/..., they do not actually trigger a download. The user has to manually save the file or right click > save as.
• Regardless of the version of the docs being browsed, the link always points to the latest version of the notebook. This means that someone browsing a specific version of the docs corresponding to their version of Mitsuba will download potentially incompatible notebooks.

Instead, we could use the :download: role in Sphinx, which makes a copy of the file to be downloaded when the documentation is built.
This means the notebooks would be hosted by ReadTheDocs, be immediately downloadable, and stay in sync.