docs: fix DatetimeOrdinal user guide and api documentation

[ankitlade12]

Fix DatetimeOrdinal User Guide and API Documentation (#898)

Description
Resolves issue #898 where the DatetimeOrdinal user guide was incorrectly displaying API information.

Changes

• User Guide (docs/user_guide/datetime/DatetimeOrdinal.rst): Removed the .. autoclass:: directive that was inadvertently injecting the API spec into the main user guide text. Added an explanation detailing why converting datetime features into ordinal variables is useful for machine learning models (i.e., preserving relative temporal distances while dropping formatting complexities).
• API Documentation (docs/api_doc/datetime/DatetimeOrdinal.rst): Updated the directive from .. automodule:: to .. autoclass:: feature_engine.datetime.DatetimeOrdinal to correctly surface the transformer methods and documentation in the proper location.

Checklist

• I have formatted the code using black and isort
• I have run flake8 and mypy and the build passes
• I have passed pytest locally
• I have updated the documentation accordingly

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.27%. Comparing base (c02fc7a) to head (2dc1603).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #899   +/-   ##
=======================================
  Coverage   98.27%   98.27%           
=======================================
  Files         116      116           
  Lines        4978     4978           
  Branches      795      795           
=======================================
  Hits         4892     4892           
  Misses         55       55           
  Partials       31       31           

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