[docs] Mobile doc updates (#21605)
### Description
<!-- Describe your changes. -->
- Update Android build instructions.
- Remove references to "mobile" packages with reduced operators and
minimal build which have been removed in 1.19.
In particular, these packages:
- onnxruntime-mobile-c/objc (iOS)
- onnxruntime-mobile (Android)
https://edgchen1.github.io/onnxruntime/docs/
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
Update and remove obsolete documentation.
