Update Makefile to automatically run post-build script for thumbnails

- Modified html target to run post_build.py after sphinx build
- Ensures animated GIF thumbnails are processed in local builds
- Matches the workflow that will be used on Read the Docs
- Complete end-to-end solution verified with clean build

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: jeremymanning

docs/Makefile

@@ -46,6 +46,8 @@ notebooks:
 
 html:
 	$(SPHINXBUILD) -b html -E $(ALLSPHINXOPTS) $(BUILDDIR)/html
+	@echo "Running post-build script to fix animated thumbnails..."
+	python post_build.py
 	@echo
 	@echo "Build finished. The HTML pages are in $(BUILDDIR)/html."