Publish sources jar and javadoc jar

Currently, IntelliJ and other IDEs aren't able to automatically jump to source
or documentation for this library, since those jars aren't published.

We should get a nice developer experience improvement by providing these jars.
This follows the example configuration in
https://github.com/jitpack/maven-simple

Author: jklukas

core/pom.xml

@@ -150,6 +150,30 @@
                     <autoReleaseAfterClose>true</autoReleaseAfterClose>
                 </configuration>
             </plugin>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-source-plugin</artifactId>
+                <executions>
+                    <execution>
+                        <id>attach-sources</id>
+                        <goals>
+                            <goal>jar</goal>
+                        </goals>
+                    </execution>
+                </executions>
+            </plugin>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-javadoc-plugin</artifactId>
+                <executions>
+                    <execution>
+                        <id>attach-javadocs</id>
+                        <goals>
+                            <goal>jar</goal>
+                        </goals>
+                    </execution>
+                </executions>
+            </plugin>
         </plugins>
     </build>
     <dependencies>