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: Id1afd8434f59437a9ef0b5834fb8f9aa17f804e1
(cherry picked from commit 769a78d7068d4d64e250cdc6e70b5c9480ef34bb)
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