docs(readme): project name from Univer MCP to Univer Platform

[DR-Univer]

close #xxx

Pull Request Checklist

• Related tickets or issues have been linked in the PR description (or missing issue).
• Naming convention is followed (do please check it especially when you created new plugins, commands and resources).
• Unit tests have been added for the changes (if applicable).
• Breaking changes have been documented (or no breaking changes introduced in this PR).

[github-actions]

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📑 Demo (React@19)	Demo (React@16)	📚 Storybook
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[github-actions]

Playwright test results

 26 passed

Details

 26 tests across 12 suites
 6 minutes, 12 seconds
 fdb1347
 For more information, see full report

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 33.22%. Comparing base (1748f5f) to head (fdb1347).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #6063      +/-   ##
==========================================
- Coverage   33.24%   33.22%   -0.02%     
==========================================
  Files        2763     2763              
  Lines      145758   145758              
  Branches    32531    32531              
==========================================
- Hits        48453    48435      -18     
- Misses      97305    97323      +18     

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