Add Gitea workflow publishing the library to the public Maven registry

- .gitea/workflows/publish.yml: build, test and deploy on push to main
  (snapshot), on v* tags (release version from the tag) and manually;
  authenticates with the REGISTRY_TOKEN secret
- demo app is excluded from deployment
- pin maven-deploy-plugin and versions-maven-plugin
- README: how to consume the package

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G71TQ5jDETVwdUJxGAwaiT

mailsender-demo/pom.xml

@@ -13,6 +13,11 @@
     <artifactId>mailsender-demo</artifactId>
     <description>Demo app: looks up the recipient's WKD key and sends test e-mails</description>
 
+    <properties>
+        <!-- sample app, not a library: never published -->
+        <maven.deploy.skip>true</maven.deploy.skip>
+    </properties>
+
     <dependencies>
         <dependency>
             <groupId>pro.thinhha.mailsender</groupId>