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

- 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.

Author: issyl0

Library/.rubocop.yml

@@ -31,6 +31,7 @@ AllCops:
     - "Homebrew/bin/*"
     - "Homebrew/vendor/**/*"
     - "Taps/*/*/vendor/**/*"
+    - "**/.github/copilot-instructions.md" # This should be treated the same as the `docs/` files.
   SuggestExtensions:
     rubocop-minitest: false