Update guide to UDFs with notes about Series.applymap deprecation a…

…nd related changes (#10607)

This PR updates our guide to UDFs notebook in the following ways:

- Notes deprecation of `cudf.Series.applymap`
- reorder sections such that `cudf.Series.apply` and `cudf.DataFrame.apply` are encountered before `applymap`, `apply_rows` and `apply_chunks`
- Adds recommendations about use cases for each of the above APIs to distinguish them
- Minor updates all over to better describe current state

EDIT: decided to just remove the docs for `applymap` at this point.

Authors:
  - https://github.com/brandon-b-miller

Approvers:
  - Ashwin Srinath (https://github.com/shwina)

URL: #10607