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

pom.xml

@@ -93,6 +93,16 @@
                     <artifactId>maven-surefire-plugin</artifactId>
                     <version>3.5.3</version>
                 </plugin>
+                <plugin>
+                    <groupId>org.apache.maven.plugins</groupId>
+                    <artifactId>maven-deploy-plugin</artifactId>
+                    <version>3.1.4</version>
+                </plugin>
+                <plugin>
+                    <groupId>org.codehaus.mojo</groupId>
+                    <artifactId>versions-maven-plugin</artifactId>
+                    <version>2.18.0</version>
+                </plugin>
             </plugins>
         </pluginManagement>
         <plugins>