doc: add sphinx-confluencebuilder-bridge reference

Adding a reference to the sphinx-confluencebuilder-bridge extension for
an alternative way of handling Confluence directives/etc. in other
builders.

Signed-off-by: James Knight <git@jdknight.me>

Author: jdknight

doc/directives.rst

@@ -15,6 +15,12 @@ used to restrict these directives to supported builders by using the
         .. confluence_metadata::
             :labels: label-test
 
+Alternatively, users may be interested in a third-party extension which can
+help support the use of Confluence directives in other builders:
+
+    | sphinx-confluencebuilder-bridge
+    | https://github.com/adamtheturtle/sphinx-confluencebuilder-bridge
+
 Common
 ------