Add better native nuget package readme (#21889)
### Description
<!-- Describe your changes. -->
Request from Nuget team to add a better readme to the nuget package so
it is displayed nicely on nuget.org.
Previously we were using the ORT repo readme.md but that a) doesn't
display correctly due to limited markdown support on nuget.org, and b)
has a lot of irrelevant info like build pipeline status.
- Created a generic readme.md that includes the ORT description from the
main readme, includes the ORT logo via an acceptable link, and lists the
native nuget packages so the file can be included in any of them as-is.
- Updated the nuget packaging script to add the `readme` tag and use
this file.
### 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. -->
Request from MS Nuget team to MS package owners to add.
