Point to control flow docs in core pytorch (#1083)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1083

Stop referring to the deprecated `docs/website` tree. Pytorch core already provides some information about control flow concepts, so we can point to that instead.

While I'm here, remove stray blank lines.

Reviewed By: mergennachin

Differential Revision: D50612692

fbshipit-source-id: a56d2127cf7a0d29538b9943c180075c5f2b25f1