Add manual NuGet publishing workflow

[Benziza]

Summary

• Add a manually triggered NuGet.org publishing workflow that runs tests and packages the tool before pushing.
• Require the repository's NUGET_API_KEY secret and only allow publishing from main.
• Document the release steps outside the short user README.

Verification

• Local tests pass (10 total) and dotnet pack creates WhyConfig.NET.0.1.0.nupkg.
• The package installs from a local feed and runs whyconfig --help.
• Publishing is not triggered by this PR.