DOC: Updated Resampler.mean docstring to have its own docstring
- [x] closes #48803 which belongs to `Contributions welcome` milestone
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.


By default, the `Resampler.mean` docstring is inherited from `GroupBy.mean`. As a result, unsupported parameters 'engine' and 'engine_kwargs' were created in the [API documentation](https://pandas.pydata.org/docs/reference/api/pandas.core.resample.Resampler.aggregate.html).

To do this, I defined the Mean function as a method of Resampler and wrote the docstring of the function itself.

- FYI
  - I think it is unnecessary adding this issue to Whatsnew section, so I left it.
  - I closed [previous PR](https://github.com/pandas-dev/pandas/pull/48902). This one is newly created.
I found that doc build error also occurs in previous commit(#48965).
when I run `python doc/make.py html` in my local computer, same error occurs. Thanks @donghyyun 