docs: Update README for batch check

[jimmyjames]

Description

Updates the docs in the README for batchCheck, to document the new server-side functionality, and reference clientBatchCheck for those on an earlier version of OpenFGA.

References

Review Checklist

• I have clicked on "allow edits by maintainers".
• I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• The correct base branch is being used, if not main
• I have added tests to validate that the change in functionality is working as expected

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.01%. Comparing base (571b53f) to head (9140299).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #197   +/-   ##
=======================================
  Coverage   88.01%   88.01%           
=======================================
  Files          23       23           
  Lines        1202     1202           
  Branches      192      211   +19     
=======================================
  Hits         1058     1058           
  Misses         85       85           
  Partials       59       59           

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