Add a subsection about the MTU workaround
Workaround and guidance was document in https://gitlab.com/gitlab-com/gl-infra/production/-/work_items/22051#note_3347244335. Let's add that to the docs near https://docs.gitlab.com/runner/executors/docker/#overriding-the-mtu-of-the-created-network.
_This ticket was created from_ [_INC-10073_](https://app.incident.io/gitlab/incidents/10073) _using_ [_incident.io_](https://app.incident.io) 🔥
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