docs: introduce GCS backend docs

[avriiil]

This PR adds a documentation page for using delta-rs with the GCS storage backend

[github-actions]

ACTION NEEDED

delta-rs follows the Conventional Commits specification for release automation.

The PR title and description are used as the merge commit message. Please update your PR title and description to match the specification.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.50%. Comparing base (7783f66) to head (e9a2e5f).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2903      +/-   ##
==========================================
- Coverage   72.50%   72.50%   -0.01%     
==========================================
  Files         131      131              
  Lines       40421    40421              
  Branches    40421    40421              
==========================================
- Hits        29309    29308       -1     
  Misses       9238     9238              
- Partials     1874     1875       +1     

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