Add readthedocs-cli to requirements.in

This will allow us to manage our redirects in-tree.

Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>
diff --git a/docs/requirements.in b/docs/requirements.in
index a523188..14d618c 100644
--- a/docs/requirements.in
+++ b/docs/requirements.in
@@ -1,2 +1,3 @@
-sphinx-rtd-theme
 breathe
+readthedocs-cli
+sphinx-rtd-theme