build(dcl): add Gitea feed + ZB.MOM.WW.MxGateway.Client package reference

Central package management requires package-source mapping with >1 feed
(NU1507 as error), so nuget.config scopes ZB.MOM.WW.MxGateway.* to the Gitea
feed and everything else to nuget.org. Credentials are not committed.

nuget.config

@@ -0,0 +1,27 @@
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+  <packageSources>
+    <clear />
+    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
+    <add key="dohertj2-gitea" value="https://gitea.dohertylan.com/api/packages/dohertj2/nuget/index.json" />
+  </packageSources>
+  <!--
+    Central package management requires explicit source mapping when more than
+    one feed is defined: nuget.org serves everything; the Gitea feed serves only
+    the ZB.MOM.WW.MxGateway.* packages.
+  -->
+  <packageSourceMapping>
+    <packageSource key="nuget.org">
+      <package pattern="*" />
+    </packageSource>
+    <packageSource key="dohertj2-gitea">
+      <package pattern="ZB.MOM.WW.MxGateway.*" />
+    </packageSource>
+  </packageSourceMapping>
+  <!--
+    Credentials are NOT committed. Provide them per-developer by running
+    `dotnet nuget add source` for the Gitea feed (with username + access token
+    and the store-password-in-clear-text flag), or supply NuGet credential
+    env vars in CI / the docker build (see docker/deploy.sh wiring in Task 19).
+  -->
+</configuration>