chore: Add dedicated environment for release.yml workflow

[filzrev]

This PR intended to isolate release.yml workflow with other workflows.
It's required because release.yml using secrets. that should not be exposed to public.

Required steps before running updated release workflow

1. Create new release environment at https://github.com/dotnet/docfx/settings/environments
2. Create new secrets for NuGet package sign/publish.
3. Register new secrets to release environment
4. Invalidate old secrets. And remove secrets from other environments/repository.

[Copilot]

Pull Request Overview

This PR isolates the release workflow by adding a dedicated environment to better manage secrets.

• Added "environment: release" field to the release.yml workflow.
• Ensured that the release process uses new secrets and is segregated from other workflows.