Prefill release notes URL field

[hugovk]

Description

• When creating a new release, prefill the release notes URL field
• Use What's New for pre-releases:
  • "Python 3.15.0a1" => https://docs.python.org/3.15/whatsnew/3.15.html
• Use the changelog for regular releases:
  • "Python 3.13.9" => https://docs.python.org/release/3.13.9/whatsnew/changelog.html
• When editing an existing release, don't change it

Closes

• Fixes Autofill release notes URL release-tools#276.