[Security][Alerting]: Add docs for installing and updating prebuilt rules in air-gapped environments

[nastasha-solomon]

Summary

This PR creates a dedicated page for installing and updating Elastic prebuilt detection rules in air-gapped environments. The following methods are covered:

• Using a self-hosted Package Registry (recommended)
• Manually transferring prebuilt rules using the export/import process

Fixes:

• Add information for airgapped environments about upgrade process of Security prebuilt detection rules security-docs#4652
• Document how to host/import prebuilt rules in air-gapped environments security-docs#2932

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

Cursor, claude-4.5-opus-high

[github-actions]

Vale Linting Results

Summary: 1 suggestion found

💡 Suggestions (1)

File	Line	Rule	Message
solutions/security/detect-and-alert/prebuilt-rules-airgapped.md	170	Elastic.Wordiness	Consider using 'drag' instead of 'Drag and drop'.

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[github-actions]

🔍 Preview links for changed docs

• solutions/security/detect-and-alert/prebuilt-rules-airgapped.md
• solutions/security/detect-and-alert/install-manage-elastic-prebuilt-rules.md