Add initial API doc configuration

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/docs/requirements.in b/docs/requirements.in
new file mode 100644
index 0000000..c0afd61
--- /dev/null
+++ b/docs/requirements.in
@@ -0,0 +1,3 @@
+sphinx-rtd-theme
+breathe
+exhale