fix: update readme docs

[elribonazo]

Explanation

Simple PR just including docs

References

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've highlighted breaking changes using the "BREAKING" category above as appropriate

---

Note

Updates READMEs to add build/dev/test instructions and streamline the playground docs.

• Docs:
  • packages/sdk-multichain-ui/README.md: Add package overview and usage, including development (yarn dev/yarn start) and build instructions.
  • packages/sdk-multichain/README.md: Add local build guidance (requires building sdk-multichain-ui first) and testing commands (yarn test, yarn test:ci).
  • playground/multichain-react/README.md: Simplify by removing environment configuration/Helius sections; keep installation and usage basics.

^{Written by Cursor Bugbot for commit e1d7fe9. This will update automatically on new commits. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.99%. Comparing base (ac8f8c3) to head (e1d7fe9).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1354   +/-   ##
=======================================
  Coverage   74.99%   74.99%           
=======================================
  Files         184      184           
  Lines        4519     4519           
  Branches     1108     1108           
=======================================
  Hits         3389     3389           
  Misses       1130     1130           

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud