chore: gitignore build artifacts and clean up scratch files

Add _build/, mlsysim/docs/assets/, site_libs, rendered HTML, and
test plan YAML to .gitignore. Remove leftover scratch files from root.

Author: profvjreddi

.gitignore

@@ -208,3 +208,12 @@ newsletter/drafts/*.md
 
 /.quarto/
 **/*.quarto_ipynb
+
+# Quarto / mlsysim rendered site output
+_build/
+mlsysim/docs/assets/
+mlsysim/docs/site_libs/
+mlsysim/docs/**/*.html
+
+# mlsysim scratch / test plans
+mlsysim/test_*.yaml