quant docs: add and clean up hardsigmoid (#40340)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40340
Adds and simplifies quantization docs for hardsigmoid
Test Plan:
* build docs on Mac OS
* inspect
Differential Revision: D22152634
Pulled By: vkuzo
fbshipit-source-id: 18da273023fb00e5f0bc1e881b00536492c606d3
