Docs feedback: Mattermost integration doc Links broken

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Doc: https://docs.gitlab.com/ee/user/project/integrations/mattermost.html

404 links: "2. Enable incoming web hooks" https://docs.mattermost.com/developer/webhooks-incoming.html#enabling-incoming-webhooks

"3. Add an incoming web hooks" https://docs.mattermost.com/developer/webhooks-incoming.html#creating-integrations-using-incoming-webhooks