DOC: don't teach infer_datetime_format in user_guide
- [x] edited the file `doc/user_guide/io.rst`.

Just removed the `infer_datetime_format` from the docs. 

It's my first pull request & it's not fully complete, so can you guide me on what else changes should be made.

closes #35296
@MarcoGorelli any updates? Yeah sorry if I wasn't clear enough. That's exactly what I wanted to do  @MarcoGorelli thanks! 
Had a great experience working with you and learned a lot.