Improve documentation around builtin functions (#30347)
Summary:
This breaks the builtins page into some more sections and adds details about Python built-in functions
](https://our.intern.facebook.com/intern/diff/18718166/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30347
Pulled By: driazati
Reviewed By: wanchaol
Differential Revision: D18718166
fbshipit-source-id: bf43260ab7bcf92cccef684a5ce68cb16020771d
