[WebIDE] Write admin documentation for setting up Web IDE on air-gapped instances

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Problem to solve

We don't have any documentation that explains how to set up a custom Web IDE extension host domain on Gitlab Self-Managed instances. This is particularly important for GitLab Self-Managed instancves running on air-gapped environments.

Solution proposal

After delivering [WebIDE] Use GitLab instance as the Web IDE's e... (#576648 - closed), document how instance administrators can set up a custom Web IDE extension host domain.

Acceptance Criteria

• The existing documented instructions for air-gapped instances live in a consumable way in the main product docs and have been TW reviewed and approved.
• Security review suggestion Inform GitLab instance administrators about the risk of scoping a domain too broadly when setting a custom extension host domain.