[FIX] conf: avoid // urls

e.g. https://www.odoo.com/documentation/user/13.0//crm.html when
creating link with no branch/lang name

conf.py

@@ -401,6 +401,8 @@ def localize(app, pagename, templatename, context, doctree):
 
 
 def _build_url(root, branch, pagename):
+    if not branch:
+        root = root.rstrip('/')
     return "{canonical_url}{canonical_branch}/{canonical_page}".format(
         canonical_url=root,
         canonical_branch=branch,