Fix and improve example workflow

[rufer7]

Why:

Closes: #36383 36383

What's being changed (if available, include any code snippets, screenshots, or gifs):

• Improve comments
• Add workflow_dispatch
• Specify permissions granted to GITHUB_TOKEN
• Replace $ORGANIZATION_VAR with ${{ github.repository_owner }}
• Add support for null in last_activity_at
• Replace placeholder ORG/REPO with ${{ github.repository }}
• Fix env variable (see https://docs.github.com/en/actions/security-for-github-actions/security-guides/automatic-token-authentication#example-1-passing-the-github_token-as-an-input)

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the preview environment.

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Source	Review	Production	What Changed
copilot/rolling-out-github-copilot-at-scale/reminding-inactive-users.md	fpt ghec	fpt ghec

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[sunbrye]

@rufer7 Thank you for making a contribution and working through any bugs you found on the workflow ✨ The updates look great, especially the support for null last_activity_at. I'm going to approve and merge this PR ✅

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How to review these changes 👓

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The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on the review server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
copilot/rolling-out-github-copilot-at-scale/reminding-inactive-users.md	fpt ghec	fpt ghec

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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[github-actions]

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