Add steps for reviewing pull requests locally to contribution guide

[Abod-A]

Description

This pull request adds a new step to the contribution guide (CONTRIBUTE.md) explaining how to review other contributors' pull requests locally.

The new step 11, Review Other Pull Requests Locally, covers:

• Adding the main repository as the upstream remote (one-time setup)
• Fetching a pull request into a local branch with git fetch upstream pull/<PR_NUMBER>/head:pr-<PR_NUMBER>
• Checking out the PR branch to run and test the changes locally
• Switching back to your own branch when finished
• Deleting the local PR branch afterwards (this only removes the local copy, not the PR itself)

This makes it easier for team members to properly test peer PRs (e.g. running usage examples) instead of only reading the diff on GitHub.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (update or new)

How Has This Been Tested?

The steps were tested end to end on a real peer pull request (#871):

• Fetched the PR with git fetch upstream pull/871/head:pr-871
• Checked out pr-871 and verified the PR's files appeared locally
• Switched back to my own branch and verified the PR's files were no longer in the working tree
• Deleted the local branch with git branch -D pr-871 and confirmed my own branches and uncommitted work were unaffected

Testing Checklist

• Tested in latest Chrome
• Tested in latest Firefox
• npm run build
• npm run preview
• npm run dev

Checklist

If involving code

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

Folders and Files Added/Modified

• Modified:
  • CONTRIBUTE.md

Additional Notes

The new step follows the same format as the existing steps in the guide (bold heading, bullet points, and shell code blocks).

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