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

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

[github-actions]

📊 Performance Test Results

Comparing 25e8212 vs trunk

site-editor

Metric	trunk	25e8212	Diff	Change
load	11175.00 ms	9789.00 ms	-1386.00 ms	🟢 -12.4%

site-startup

Metric	trunk	25e8212	Diff	Change
siteCreation	25348.00 ms	24300.00 ms	-1048.00 ms	🟢 -4.1%
siteStartup	7016.00 ms	9010.00 ms	+1994.00 ms	🔴 28.4%

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Results are median values from multiple test runs.

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