[Rules] Add example for subrequests from other zones

[pedrosousa]

Summary

Add new Transform Rules example adding a header for subrequests from zones other than the current one.

Based on the example in the related changelog entry.

[hyperlint-ai]

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PR Change Summary

Added an example for creating a request header transform rule for subrequests originating from different zones.

• Introduced a new example for Transform Rules regarding subrequests from other zones.
• Provided a specific expression for modifying request headers based on the upstream zone.
• Included details on setting a static header value for external subrequests.

Added Files

• src/content/docs/rules/transform/examples/add-request-header-subrequest-other-zone.mdx

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[github-actions]

This pull request requires reviews from CODEOWNERS as it changes files that match the following patterns:

Pattern	Owners
/src/content/docs/rules/	@pedrosousa, @cloudflare/pcx-technical-writing

[github-actions]

Preview URL: https://dd7d50c5.preview.developers.cloudflare.com
Preview Branch URL: https://pedro-2025-06-17-rules-add-example-subrequests.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/rules/transform/examples/add-request-header-subrequest-other-zone/	https://pedro-2025-06-17-rules-add-example-subrequests.preview.developers.cloudflare.com/rules/transform/examples/add-request-header-subrequest-other-zone/