workflow for first time contributor

[maryliag]

Adds a comment with guidance for first time contributors.
Option to add custom message at the end and to add custom label

[Copilot]

Pull request overview

This PR adds a new reusable GitHub Actions workflow that welcomes first-time contributors when they open a pull request. It posts a customizable welcome comment (contributing guidelines, CLA reminder, etiquette links) and optionally applies a label. The change follows the repository's established shared-workflow layout (flat workflow file under .github/workflows/ plus a companion root docs folder), mirroring the existing zizmor reusable workflow, and adds a catalog row to the top-level README. Security-wise it correctly avoids using PR-author-controlled content and guards pull_request_target with an org-owner check and author_association == 'NONE'.

Changes:

• New reusable workflow_call workflow .github/workflows/first-time-pr.yml with custom_message and labels inputs, org/first-time-contributor gating, and comment/label steps.
• New consumer docs first-time-pr/README.md with usage snippet, inputs table, default comment body, and firing conditions.
• New catalog row in README.md for the First-time contributor workflow.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
.github/workflows/first-time-pr.yml	Implements the reusable workflow: inputs, permissions, concurrency, gating condition, label step, and heredoc-based welcome comment.
first-time-pr/README.md	Consumer-facing documentation: usage, inputs, default comment, custom example, and firing conditions.
README.md	Adds a catalog row linking to the new workflow and its README.

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