Update README for MediaTek backend (#5386)

Summary:
1. Move common setup instructions under backends/
2. Refactoring
3. Add link for NeuroPilot Express SDK portal

Pull Request resolved: https://github.com/pytorch/executorch/pull/5386

Reviewed By: kirklandsign

Differential Revision: D63515265

Pulled By: cccclai

fbshipit-source-id: 7834397613489650a815f50212c8bb20490afafd