Add Git ignore patterns for LaTexML-generated build directories

.gitignore

@@ -151,3 +151,17 @@ LaTeXML.css
 MLS.tar.gz
 ltx-listings.css
 ltx-report.css
+
+# Directories belonging to the LaTeXML build
+/annotations/
+/connectors-and-connections/
+/interface-or-type-relationships/
+/modelica-dae-representation/
+/overloaded-operators/
+/packages/
+/preface/
+/state-machines/
+/stream-connectors/
+/synchronous-language-elements/
+/the-modelica-standard-library/
+/unit-expressions/