Add build config and test config to run documentation build

Developer is expected to install the necessary tools to build
documentation locally [1]

1. https://trustedfirmware-a.readthedocs.io/en/latest/getting_started/docs-build.html

Change-Id: Ic31c23b97d94e516d0a5b357e5e8462e38afe81f
Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>
diff --git a/tf_config/build-docs b/tf_config/build-docs
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/tf_config/build-docs