Add documentation for retries in GCSFS

[Mahalaxmibejugam]

This PR updates docs/source/retries.rst to provide comprehensive details on the default retry behavior for Standard, Hierarchical Namespace (HNS) and Zonal buckets

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.30%. Comparing base (d1bb2cc) to head (8a50436).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #789      +/-   ##
==========================================
+ Coverage   75.84%   76.30%   +0.46%     
==========================================
  Files          14       14              
  Lines        2645     2663      +18     
==========================================
+ Hits         2006     2032      +26     
+ Misses        639      631       -8     

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