Add a note for copilot-setup-steps.yml and using with a merge queue

### Code of Conduct

- [x] I have read and agree to the GitHub Docs project's [Code of Conduct](https://github.com/github/docs/blob/main/.github/CODE_OF_CONDUCT.md)

### What article on docs.github.com is affected?

https://docs.github.com/en/copilot/how-tos/use-copilot-agents/coding-agent/customize-the-agent-environment#customizing-copilots-development-environment-with-copilot-setup-steps

### What part(s) of the article would you like to see updated?

Somewhat related to #42379

In the github docs, it says that you can add the following to you copilot-setup-steps.yml file:

```yaml
name: "Copilot Setup Steps"

## Background
# Automatically run the setup steps when they are changed to allow for easy validation, and
# allow manual testing through the repository's "Actions" tab
on:
workflow_dispatch:
push:
paths:
- .github/workflows/copilot-setup-steps.yml
pull_request:
paths:
- .github/workflows/copilot-setup-steps.yml
```

While this works, I've found that if you use a merge queue then using the above configuration results in your workflow running every time a PR is added to the merge queue. We are use the larger, custom image GitHub hosted runners and this results in unnecessary runs & costs.

[github/copilot-sdk](https://github.com/github/copilot-sdk/actions/workflows/copilot-setup-steps.yml?query=event%3Apush) was the first public GitHub repo I found that had the same issue(although this one is just using default ubuntu-latest runners). It helped me figure out that behavior wasn't unique to our setup

## Proposed Content Update

I'd like to update the docs to either add an inline comment with commented out code or add a little inline note about if using a merge queue. This is what ended up working for us:

```yaml
name: "Copilot Setup Steps"

# Automatically run when changed so setup issues surface immediately.
# Also allows manual testing via the "Actions" tab.
on:
workflow_dispatch:
push:
branches-ignore:
- 'gh-readonly-queue/**'
paths:
- .github/workflows/copilot-setup-steps.yml
pull_request:
branches:
- main
paths:
- .github/workflows/copilot-setup-steps.yml
```

### Additional information

_No response_