doc: conf: Add 2.3.0 version selector

Add the 2.3.0 release to the version selector for the documentation.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
diff --git a/doc/conf.py b/doc/conf.py
index a421f46..35376ff 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -539,6 +539,7 @@
     'theme_logo_only': False,
     'current_version': version,
     'versions': (("latest", "/"),
+                 ("2.3.0", "/2.3.0/"),
                  ("2.2.0", "/2.2.0/"),
                  ("2.1.0", "/2.1.0/"),
                  ("2.0.0", "/2.0.0/"),