Add a redirect stub for the Hipchat page

This page was removed suddenly, and no redirect stub was put in
its place.

doc/user/project/integrations/hipchat.md

+---
+redirect_to: 'index.md'
+---
+
+This document was moved to [another location](index.md).
+<!-- This redirect file can be deleted after 2021-06-30. -->
+<!-- Before deletion, see: https://docs.gitlab.com/ee/development/documentation/#move-or-rename-a-page -->