Add documentation for compile API (#27114)
### Description
Preview:
https://adrianlizarraga.github.io/onnxruntime/docs/execution-providers/EP-Context-Design.html#compile-api
Adds documentation for the ORT compile API. Includes the following
examples:
- Compiling to an output stream with custom function that allows an
application to specify where each initializer is stored.
- Cross-compiling with plugin EPs
- EPContext weight sharing with plugin EPs
### 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. -->
