keyv - feat: adding in BigMap in Keyv documentation

[jaredwray]

Please check if the PR fulfills these requirements

• Followed the Contributing guidelines.
• Tests for the changes have been added (for bug fixes/features) with 100% code coverage.
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
keyv - feat: adding in BigMap in Keyv documentation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (31e819d) to head (d4cad36).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1657   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           30        30           
  Lines         3735      3735           
  Branches       970      1025   +55     
=========================================
  Hits          3735      3735           

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