DEPR: Positional arguments in Series.to_markdown
- [X] closes #57280
- [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.
- [X] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

@mroeschke Pinging on green Looks good to me!

If you want to, you can also add overloads that already use the keyword-only syntax:


```py
    @overload
    def to_markdown(
        self,
        buf: None = ...,
        *,
        mode: str = ...,
        index: bool = ...,
        storage_options: StorageOptions | None = ...,
        **kwargs,
    ) -> str: ...

    @overload
    def to_markdown(
        self,
        buf: IO[str],
        *,
        mode: str = ...,
        index: bool = ...,
        storage_options: StorageOptions | None = ...,
        **kwargs,
    ) -> None: ...

    @overload
    def to_markdown(
        self,
        buf: IO[str] | None,
        *,
        mode: str = ...,
        index: bool = ...,
        storage_options: StorageOptions | None = ...,
        **kwargs,
    ) -> str | None: ...
```
 The changes look good to me, but the doc-test is failing (@mroeschke). The doc-test should definitely succeed before merging as the typing tests run after the doc-tests (in case mypy/pyright pick up something). Thank you @rmhowe425 !