[Docs][Github] Add initial automated first-time contributor message (#24702)

The aim is to provide common links & guidance for first-time PR or issue
creators, and to communicate our AI tool use policy. The initial
automation runs on pull requests and issues created by first-time
contributors.

This introduces an expectation for first-time contributors to
acknowledge the project policies through an explicit comment.

The workflow setup and CLI menu has been assisted by Codex and fully
reviewed. The implementation logic was further refined manually.
Naturally, the messages are fully hand-written.

Addresses #24697.

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Signed-off-by: Artem Gindinson <gindinson@roofline.ai>