[WebGPU plugin EP] Update release and packaging-related docs (#29059)
### Description
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WebGPU plugin EP doc updates:
- document release process in more detail
- fix some accuracy/clarity issues in other packaging-related docs and
comments
### 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. -->
Improve docs.
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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>