Qualcomm AI Engine Direct - refactor documents (#1168)

Summary:
- Remove duplicated document under backend/qualcomm/setup.md and point it to docs/source/build-run-qualcomm-ai-engine-direct-backend.md
- Fix typos, breakage of links and add keyword hint for QNN

Pull Request resolved: https://github.com/pytorch/executorch/pull/1168

Reviewed By: kirklandsign

Differential Revision: D53947053

Pulled By: cccclai

fbshipit-source-id: 99bbb56dbfc1365a51fe383a5f4fb9a17a3d6331