[Docs] Plugin EP packaging guidance (#26951)
### Description
<!-- Describe your changes. -->
Add initial plugin EP packaging guidance documentation.
The new page is not connected up to the rest of the site yet. That will
be done in a future PR.
### 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. -->
Provide guidance for implementers.
