docs: Require read-only mode for the registry metadata imports

What does this MR do?

The one-step import warning and step two of the three-step import now require read-only mode, which every tab already sets, and no longer offer shutting down the registry.

On a GitLab 19.4.1 instance, after gitlab-ctl stop registry, every push made during the import was accepted, and 15 of the pushed tags were missing from the database afterwards while still in storage. With read-only mode, the registry refused every push and the database matched storage.

One-step import with the registry stopped

sequenceDiagram
    actor Admin
    participant ctl as gitlab-ctl registry-database
    participant reg as registry service
    actor Client as registry client
    Admin->>reg: gitlab-ctl stop registry, then reconfigure
    Admin->>ctl: migrate up
    ctl->>reg: start! unless running?
    Admin->>ctl: import --log-to-stdout
    Client->>reg: push during the import
    reg-->>Client: accepted on legacy metadata
    Note over ctl,reg: Some pushed images stay in storage<br/>but are missing from the database

gitlab-ctl runs start! unless running? after every registry-database command that succeeds, in its source from 17.5.0 to 19.4.1.

Step two of the three-step import gets the same change, because gitlab-ctl also starts a stopped registry after step two, and the registry in GitLab 18.7.0 does not refuse that start by default.

Related issues

Related to #606166.

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This MR was prepared with assistance from Claude Code. The import behavior was observed on a GitLab 19.4.1 instance, the restart and the 18.7.0 default were read from the linked source, and both pages pass Vale and markdownlint at the CI versions.