Add support for output vars for GHA Drone Plugin step

[dewan-ahmed]

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Description

• Please describe your changes: Updates https://developer.harness.io/docs/continuous-integration/use-ci/use-drone-plugins/run-a-git-hub-action-in-cie to show output var support for GHA Drone Plugin step
• Jira/GitHub Issue numbers (if any): CI-12667, CI-15334
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[SushrutHarness]

LGTM

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Please check the Execution Link of the Pipeline for the Website Draft URL. This is located in the Preview Step behind the Harness VPN and also is available in #hdh_alerts. E.g Website Draft URL: https://unique-id--harness-developer.netlify.app. Current Draft URL is: https://67856f588eae155d8cfd3e17--harness-developer.netlify.app