docs: Add logo asset and README branding

[lan17]

Summary

• add the new GCache logo under assets/
• render the logo into assets/logo.png and keep an editable assets/logo.svg
• display the logo at the top of the README

Testing

• not run (docs/assets change only)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.47%. Comparing base (de11b6c) to head (02864f9).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #123   +/-   ##
=======================================
  Coverage   97.47%   97.47%           
=======================================
  Files          13       13           
  Lines         673      673           
=======================================
  Hits          656      656           
  Misses         17       17           

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