[codex] Add local plugin registry guidance for air-gapped setups

[llewellyn-sl]

Summary

This draft adds practical guidance for running Nextflow plugins with a custom or local registry in restricted or air-gapped environments.

Source context

• Slack source: https://seqera.slack.com/archives/C0APYR2A3A8/p1775081447163959
• Jira: https://seqera.atlassian.net/browse/EDU-1115

Doc target

• docs/plugins/plugin-registry.md

Rationale

The EDU ticket called out a missing doc path for air-gapped plugin usage. The existing docs already describe the registry itself and the relevant environment variables separately, so this change adds the minimum connective guidance in the plugin registry page: custom registry URL, local cache directory, offline mode, and when registry.nextflow.io still needs outbound access.

Validation

• Reviewed the edited page directly
• Confirmed the new guidance matches existing Nextflow environment variable docs and current plugin-registry behavior in code
• Did not run a full docs build in the temp automation clone

Follow-up

If we want a more complete air-gapped operations guide later, this section can link out to a dedicated admin-oriented page.

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