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.

CertifiEd.Client.slnx

@@ -0,0 +1,5 @@
+<Solution>
+  <Folder Name="/src/">
+    <Project Path="src/CertifiEd.Client/CertifiEd.Client.csproj" />
+  </Folder>
+</Solution>