[doc] Fix docs job by adding _static folder to avoid Sphinx warning

This adds _static/.keep to doc/ folder to stop the Sphinx error:

Warning, treated as error:
html_static_path entry '_static' does not exist

https://zuul.opendev.org/t/openstack/build/41350c344d4d45b29418a492e9b32a27

Change-Id: Iac1ee43334fe0cd68a161c1779aa308905fc6d1a
diff --git a/doc/source/_static/.keep b/doc/source/_static/.keep
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/doc/source/_static/.keep