[Quant] Add tutorial for BackendConfig (#2124)

Summary: This commit adds the tutorial for the BackendConfig,
the integration point for backend developers to specify the
quantization behavior on a given target backend.

Note to reviewers: To see the rendered version, just go to
"Files changed" > "..." > "View file".

Reviewers: jerryzh168, vkuzo

Subscribers: jerryzh168, vkuzo

Author: andrewor14