Add RLHF document

[hmellor]

Adds a documentation on RLHF which we can then add more to later.

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[youkaichao]

I would rather refer to openrlhf or verl. these are not complete examples, but just show some basic usage of how to enable rlhf.

[hmellor]

Ok, I've added references to TRL, OpenRLHF and verl. I've also qualified the examples saying that they are basic and should be used as inspiration if you don't want to use the previously mentioned libraries.