Remove pkgdown site and demo data from tracking

- Untrack R/docs/ (335 files, 69MB) — generated by CI, belongs on gh-pages only
- Untrack data/pbmc3k_raw.h5ad (5.8MB) — binary data should not be in git
- Add R/docs/ and data/ to .gitignore

Author: cafferychen777

.gitignore

@@ -19,6 +19,7 @@ R/*.png
 R/*.jpg
 R/examples/
 R/.Rcheck-ignore
+R/docs/
 
 # Python
 __pycache__/
@@ -49,6 +50,9 @@ env/
 .pytest_cache/
 .ruff_cache/
 
+# Data
+data/
+
 # Project-specific
 logs/
 cache/