Integrate release notes script into dash_site tool

[parlough]

For example, can be run like the following to generate release notes for 3.44.0:

GITHUB_TOKEN=<your_gh_token> dart run dash_site generate-release-notes \
  --release 3.44.0 \
  --previous-tag 3.41.0

[gemini-code-assist]

Code Review

This pull request introduces a new generate-release-notes command to the dash_site tool to automate generating Flutter release notes from GitHub pull requests, shares the nonEmpty utility function across commands, and updates the release notes documentation. The review feedback highlights a potential compilation error due to passing a nullable notes.body to a function expecting a non-nullable string, and suggests a cleaner approach to avoid the null-assertion operator (!) on previousLine by leveraging Dart's type promotion.

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit 84bbbf7):

https://flutter-docs-prod--docs-pr13567-feat-release-notes-gen-yhqy9rzo.web.app

[flutter-website-bot]

Staged preview of the updated flutter.dev site (updated for commit 84bbbf7):

https://flutter-dev-230821--www-pr13567-feat-release-notes-gen-za72vsxk.web.app

[abdallahshaban557]

LGTM