Docs: tell Sphinx about removed src folder

When changing the folder structure from `src/monk_tf` to `monk_tf` The
variables in Sphinx weren't changed as well. This commit repairs that problem.

See Github Issue #50.

Signed-off-by: Erik Bernoth <erik.bernoth@gmail.com>
Acked-by: Eik Binschek <binschek@dresearch-fe.de>
Acked-by: Steffen Sledz <sledz@dresearch-fe.de>