docs: Add redirect for outdated postgres upgrade guide link

Part-of: <https://dev.funkwhale.audio/funkwhale/funkwhale/-/merge_requests/2434>

docs/redirects.txt

@@ -52,3 +52,4 @@ documentation/index.html, ../contributor/documentation.html
 translators.html, ../contributor/translation.html
 federation.html, ../developer/federation/index.html
 upgrading.html, ../administrator/upgrade/index.html
+administrator_documentation/upgrade_docs/docker.html, ../../administrator/upgrade/docker.html