docs: add hudi core API docs with examples

[KnightChess]

Description

#71

• add hudi-core doc in-code

How are the changes test-covered

some doc code will be test

• N/A
• Automated tests (unit and/or integration tests)
  • Manual tests
  • Details are described below

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.79%. Comparing base (2981486) to head (703b59d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #113   +/-   ##
=======================================
  Coverage   87.79%   87.79%           
=======================================
  Files          14       14           
  Lines         729      729           
=======================================
  Hits          640      640           
  Misses         89       89           

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[KnightChess]

wait for #114 merge