Documentation: add .gitignore

The Sphinx output directory is generated.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
diff --git a/Documentation/.gitignore b/Documentation/.gitignore
new file mode 100644
index 0000000..53752db
--- /dev/null
+++ b/Documentation/.gitignore
@@ -0,0 +1 @@
+output