Add _build/ and api/ to gitignore

Signed-off-by: David Horstmann <david.horstmann@arm.com>

docs/.gitignore

diff --git a/docs/.gitignore b/docs/.gitignore
index 23f832b..11f197b 100644
--- a/docs/.gitignore
+++ b/docs/.gitignore
@@ -1,2 +1,4 @@
 *.html
 *.pdf
+_build/
+api/