Create a skill for AI-assisted onboarding

[v-dunjadenic]

Description

Summary:

• Added SKILL.md to document the repository onboarding skill and provide guided contributor support.
• Updated CONTRIBUTING.md and copilot-instructions.md for improved accuracy, clearer guidance, and better context around repo workflows and assistant behavior.

Motivation and context:

The new onboarding skill helps new contributors understand repo structure, build flow, and contribution expectations.
The documentation updates ensure contributor-facing guidance is aligned with current repo conventions and reduces confusion when using Copilot-assisted workflows.

Related issue: TODO (hosted-runner-images issue #845)

Check list

• Related issue / work item is attached
• Tests are written (if applicable)
• Documentation is updated (if applicable)
• Changes are tested and related VM images are successfully generated

[v-dunjadenic]

Regarding changes to this file:

Copilot identified these as discrepancies to the actual file structure and naming, which I verified.

I believe these changes are in scope of this PR since this document will be used for additional context by the skill and we don't want outdated info, but I understand it's kind of in a gray area so if the team agrees it's out of scope, I can remove it.

[v-dunjadenic]

I tried to follow the convention regarding branch naming but I'm unsure how it will look like when merged. I'm worried I will have a double v-dunjadenic prefix, so if the team thinks it's needed I can re-create the branch.