Add documentation about chat integration notification filters

Patrick Talbertrequested to merge
gitlab-community/gitlab-org/gitlab:ptalbert/chat_notification_docs into master

What does this MR do and why?

Add documentation about chat integration notification filters

Signed-off-by: Patrick Talbert ptalbert@redhat.com

References

Depends on: !224871
Relates to: &21117

Edited by Patrick Talbert

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