[Security] Doc how to import and upgrade prebuilt rules in an air-gapped deployment

[nastasha-solomon]

Issue

Docs for importing and updating prebuilt rules into an air-gapped are missing.

What's needed

• New 8.x and 9.x page: Would need to create a new page for the 8.x and 9.x series. Don't think these instructions would apply to Serverless users (need to verify).
• Update current docs for using prebuilt rules: The current docs don't mention that it's possible to import or update prebuilt rules in air-gapped envs. Need to add that info to: https://www.elastic.co/docs/solutions/security/detect-and-alert/install-manage-elastic-prebuilt-rules and https://www.elastic.co/docs/solutions/security/detect-and-alert/prebuilt-rules-update-modified-unmodified.

To take into account

• Will assistance from Dev and Product to test instructions. Don't know how to set up an air-gapped env to test docs. (Extensive QA testing has been done, but will still need to run through instructions myself.)

Existing related issues

• 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