Fix readme page (#7659)
* Delete mobile page
Moved to: https://www.onnxruntime.ai/docs/how-to/deploy-on-mobile.html
* Delete ONNX_Runtime_Mobile_NNAPI_perf_considerations.md
Moved to: https://www.onnxruntime.ai/docs/reference/execution-providers/NNAPI-ExecutionProvider.html#performance-tuning
* Fix links to website docs
* Update some summary text
* Add space
