Add documentation for testing with local Playground packages

[bcotrim]

Related issues

• N/A

Proposed Changes

• Add new documentation file docs/testing-with-local-playground.md with step-by-step guide for testing Studio with locally built WordPress Playground packages
• Update CONTRIBUTING.md to link to the new documentation

Testing Instructions

• Follow the documentation steps to set up local Playground packages
• Verify Studio runs with the local packages
• Make a change to a Playground package and confirm it's reflected in Studio

Pre-merge Checklist

• [x] Have you checked for TypeScript, React or other console errors?

[github-actions[bot]]

📊 Performance Test Results

Comparing 9fb258a2769fa1df7e24e346e4cab028601dcc9a vs trunk

site-editor

Metric	trunk	9fb258a2769fa1df7e24e346e4cab028601dcc9a	Diff	Change
load	10477.00 ms	9921.00 ms	-556.00 ms	🟢 -5.3%

site-startup

Metric	trunk	9fb258a2769fa1df7e24e346e4cab028601dcc9a	Diff	Change
siteCreation	16316.00 ms	19561.00 ms	+3245.00 ms	🔴 19.9%
siteStartup	5965.00 ms	6966.00 ms	+1001.00 ms	🔴 16.8%

---

Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change

[bcotrim]

Thanks for the review @wojtekn! I've addressed the comments:

• Grouped Playground packages at the end of package.json for easier replacement
• Removed the rs command reference
• Updated revert instructions to include package-lock.json and Playground repo

Can you take another look?