rubocop: Ignore .github/copilot-instructions.md under AllCops

[issyl0]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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• We use the rubocop-md extension to lint the Ruby within our Markdown files. It parses Ruby code snippets out of Markdown files and runs RuboCop on them.
• The docs/ directory has its own RuboCop configuration to handle which rules we enforce.
• Copilot's instructions are made up of all our docs. Instead of adding a separate RuboCop config in places where we want Copilot instructions files to exist but not spew linting errors (say, Taps/homebrew/homebrew-core/.github/.rubocop.yml), let's ignore this Markdown file entirely.
• Needed for Add Copilot instructions homebrew-core#250521 to pass brew style homebrew/core.

[Copilot]

Pull Request Overview

This PR excludes Copilot instructions files from RuboCop linting to prevent unnecessary style violations in documentation that incorporates code from various sources. The change allows .github/copilot-instructions.md files across the codebase to be treated consistently with other documentation files that have relaxed linting rules.

Key Changes:

• Added .github/copilot-instructions.md to RuboCop's exclusion list under AllCops

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_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[p-linnane]

Thanks Issy!

[MikeMcQuaid]

Thanks @issyl0!