Add external_url config to Sidekiq nodes

What does this MR do?

It is necessary to configure external_url on dedicated Sidekiq nodes.

If any jobs require the SSH or HTTP path of the GitLab instance and external_url is absent, GitLab will return the Sidekiq hostname instead of the domain that is used to access GitLab - also see https://gitlab.com/gitlab-org/gitlab/-/blob/v14.3.2-ee/lib/gitlab/repository_url_builder.rb#L19-27 for the central point where this is determined.

See these tickets for examples where a missing external_url on the Sidekiq node caused issues:

• https://gitlab.zendesk.com/agent/tickets/242624 - Customer had webhooks on their project configured. When an event was triggered, the webhook payload contained URLs that referenced the Sidekiq hostname rather than the external domain used to access the GitLab UI.
• https://gitlab.zendesk.com/agent/tickets/172237 - Customer was referencing JIRA issue IDs in their commits. GitLab was sending URLs that referenced the Sidekiq hostname rather than the external domain used to access the GitLab UI back to JIRA. JIRA was then referencing the wrong URL back to GitLab.

After adding external_url on the Sidekiq node, the problems were resolved.

Let's add this to our documentation so it doesn't catch anyone else out.

Related issues

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