[ADD] developer/reference: add a reference page on performance

closes odoo/documentation#2794

X-original-commit: 4d8e3f9785
Signed-off-by: Antoine Vandevenne (anv) <anv@odoo.com>
Signed-off-by: Xavier Dollé (xdo) <xdo@odoo.com>
Co-authored-by: Antoine Vandevenne (anv) <anv@odoo.com>

content/developer/reference/backend.rst

@@ -14,6 +14,7 @@ Python framework
     backend/module
     backend/reports
     backend/security
+    backend/performance
     backend/testing
     backend/http
     backend/mixins