docs: add external contributor PR advice

[MikeMcC399]

• Closes Extend CONTRIBUTING for actions after PR submission #29671

Additional details

The CONTRIBUTING > Pull Requests documentation section did not describe how an external contributor should respond to GitHub PR status messages caused by the base branch (usually develop) moving on, causing out-of-date or conflict messages.

This documentation gap is now closed with additional text.

Steps to test

N/A documentation change only.

How has the user experience changed?

Contributor process clarification only. No end-user experience change.

PR Tasks

• [na] Have tests been added/updated?
• [na] Has a PR for user-facing changes been opened in cypress-documentation?
• [na] Have API changes been updated in the type definitions?

[cypress-app-bot]

• Create a Draft Pull Request if your PR is not ready for review. Mark the PR as Ready for Review when you're ready for a Cypress team member to review the PR.

[MikeMcC399]

@jennifer-shehane

Please feel to modify any of the text if I got it wrong! 🙂

[MikeMcC399]

Haha - Proof of Concept! This PR is now in out-of-date status! Since the CI run has not hit the approval gate, I'm going to update it with the "Update branch" button.

[MikeMcC399]

... and again for the same reason

[MikeMcC399]

Update branch once more. Status is currently showing
2 skipped, 28 successful, 28 expected, 1 in progress, and 1 pending checks

[jennifer-shehane]

@MikeMcC399 Yah I don't typically run the tests for docs changes. It's pretty heavy handed.

[MikeMcC399]

@jennifer-shehane

Yah I don't typically run the tests for docs changes. It's pretty heavy handed.

Thanks for that feedback! I was more going through the process to see if I could sensibly follow the rules I had written up. They don't cover every eventuality, but I think they are helpful as guidance.