Strip notebook outputs + install nbstripout git filter

Outputs are automatically stripped on git add via .gitattributes filter.
Notebooks checked out from git will have no outputs — run them fresh.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: abossard

.gitattributes

@@ -0,0 +1,3 @@
+*.ipynb filter=nbstripout
+*.zpln filter=nbstripout
+*.ipynb diff=ipynb