[Docs] Warn against execution of arbitrary code in use of extension types

### Describe the enhancement requested

Add a warning box to the extension types docs pages to advise against use of pickle (or other frameworks or methods that can execute arbitrary code) in extension types.

The warning could be added to these pages:
- https://arrow.apache.org/docs/python/extending_types.html
- https://arrow.apache.org/docs/format/Columnar.html#format-metadata-extension-types

### Component(s)

Documentation