Document registry access and credentials for flow executor images

What does this MR do?

Neither page says what a flow's CI/CD job needs to pull its executor image from a project's container registry on the same instance, or where the pull credential can live. I hit both on a self-managed instance while writing up #598540, and the failure is a registry 401 with nothing on the GitLab side, so an administrator following the offline page has no way to read it.

The flow images page gains two sections under its offline heading: the access an internal host project has to grant (the flow's service account is external, so the internal-visibility path doesn't apply to it until either the External checkbox is cleared or the account is a member with the calling project allowlisted), and the runner-side DOCKER_AUTH_CONFIG form, since a CI/CD variable of that name never reaches a flow's job. The offline deployment page links to both from its flow execution steps.

Everything stated was measured on a self-managed 19.3.2-ee instance with real flows; the matrix is in my comment on the issue. I kept the policy options to an internal host because that's what I ran. UI labels checked against permissions/constants.js and admin/users/_access.html.haml.

Prepared with help from Claude Code and reviewed against source and the documentation style guide.

Related issues

Related to #598540

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