[Quant][docs] Add README for BackendConfig (#86523)
Summary: This adds a README for `torch.ao.quantization.backend_config`
that describes both the high level motivation and the specifications
of the BackendConfig API.
Reviewers: jerryzh168, vkuzo
Subscribers: jerryzh168, vkuzo
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86523
Approved by: https://github.com/jerryzh168
