Add bounds to sphinx requirement

As ONAP was created before Oct 20 2020 in RTD, we
must set bounds to enforce a compatible Sphinx version
(see manual).

Issue-ID: DOC-782

Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
Change-Id: I7afd592c5637f1d6300ae3070bcb28d191dc3895
diff --git a/docs/requirements-docs.txt b/docs/requirements-docs.txt
index 74a3b7a..3b3441a 100644
--- a/docs/requirements-docs.txt
+++ b/docs/requirements-docs.txt
@@ -1 +1,3 @@
 lfdocs-conf
+sphinx>=4.2.0  # BSD
+sphinx-rtd-theme>=1.0.0  # MIT