Example plugin EPs - readme and minor updates (#26498)
### Description
<!-- Describe your changes. -->
Example plugin EP updates:
- Add readme file.
- Fix some potential `OrtStatus*` leaks.
- Remove unused `DeferOrtRelease` helper.
### 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. -->
Add readme. Some clean up.
