docs: add firestore_fn module to docs gen

Author: Salakar

docs/generate.sh

@@ -86,6 +86,7 @@ PY_MODULES='firebase_functions
             firebase_functions.core
             firebase_functions.db_fn
             firebase_functions.eventarc_fn
+            firebase_functions.firestore_fn
             firebase_functions.https_fn
             firebase_functions.identity_fn
             firebase_functions.options