Minor: exclude directory with the generated stdlib documentation

(cherry picked from commit a66fc9043437d2e75f04feadcfc63c61b04bd196)
diff --git a/libraries/.gitignore b/libraries/.gitignore
index f7f4ca2..e2ae231 100644
--- a/libraries/.gitignore
+++ b/libraries/.gitignore
@@ -1,4 +1,5 @@
 target
+/doc/
 .idea
 *.iml
 *.ipr