chore: extract the SDK into its own repository

Standalone home for the CertifiEd .NET client SDK, split out of the
platform repository with its history.

- src/ layout with its own solution, Directory.Build.props and
  global.json (warnings as errors, XML docs required).
- Gitea Actions CI: build and pack on every push, publish to nuget.org
  on a v* tag using the NUGET_API_KEY secret.
- CONTRIBUTING documents the release flow and the one-time CI setup.

.gitignore

@@ -0,0 +1,8 @@
+bin/
+obj/
+*.user
+.vs/
+.idea/
+artifacts/
+*.nupkg
+*.snupkg