Revert D31447611: Quantization documentation: move backend section down
Test Plan: revert-hammer
Differential Revision:
D31447611 (https://github.com/pytorch/pytorch/commit/309a8cf46c2b551d04d03df1d6181e0c350356dc)
Original commit changeset: 537b146559bc
fbshipit-source-id: c400aef9a2ea5d18f8076879fe6354be7a6732f1
