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
• [x] Automated tests (unit and/or integration tests)
  • [ ] Manual tests
  • [ ] Details are described below

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

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

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.