DOC: Restructure and expand UDF page
I changed the order in which the methods are presented,both in the table and in the sections, to be:

- map
- apply
- pipe
- filter
- agg
- transform

I find it easier to explain them in this order.

And I expanded the method sections with examples and a bit more of information.

I removed the most complex example in the intro, as I think the examples in the sections will make a better job now at explaining the most complex cases.

@arthurlw @rhshadrach do you mind having a look?
Looks good to me! I think the example under vectorized operations should be changed to fit with the Fahrenheit example, but that can be added in a follow-up PR. Thanks @arthurlw, great feedback. I'll leave the example on the vectorized section for now, as it may make sense to also expand that section as we make progress with the IT engines. Feel free to update it now if you want, but I'm unsure at this point how to add the JIT engines to that section, and how to better present all the performance related topics. Maybe we can just add a section for it, but maybe we can find a way to present it so one topic expands on the previous, as I tried to do with the different methods.  Merging here, as I want to add few more things to this page. Please let me know if any comment, happy to address feedback in a follow up PR.