Javadoc: Add offline link to local aggregate javadocs

Assuming local docs/apidocs are up-to-date (see "Update site Apidocs" in
PUBLISHING.md), building the individual modules with javadoc:

    mvn package -P release

should create the necessary online links to any new resources/classes,
not published already.

Add some .mvn/ configuration (maven-enforcer-extension) – the presence
of that directory is required to use ${maven.multiModuleProjectDirectory}
reference in POM.

Author: stanio

.mvn/enforcer-extension.xml

@@ -0,0 +1,16 @@
+<extension>
+    <executions>
+        <execution>
+            <configuration>
+                <rules>
+                    <requireMavenVersion>
+                        <version>3.8.8</version>
+                    </requireMavenVersion>
+                    <requireJavaVersion>
+                        <version>21</version>
+                    </requireJavaVersion>
+                </rules>
+            </configuration>
+        </execution>
+    </executions>
+</extension>

.mvn/extensions.xml

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<extensions xmlns="http://maven.apache.org/EXTENSIONS/1.1.0"
+        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+        xsi:schemaLocation="http://maven.apache.org/EXTENSIONS/1.1.0
+                            https://maven.apache.org/xsd/core-extensions-1.1.0.xsd">
+    <extension>
+        <groupId>org.apache.maven.extensions</groupId>
+        <artifactId>maven-enforcer-extension</artifactId>
+        <version>3.4.1</version>
+    </extension>
+</extensions>

.mvn/readme.txt

@@ -0,0 +1 @@
+The .mvn directory is needed to be able to use the ${maven.multiModuleProjectDirectory} property.

pom.xml

@@ -187,6 +187,12 @@
                                 ><strong><a href="]]>${project.scm.url}<![CDATA[" style="text-transform: none; font-style: italic"
                                 >]]>${project.name} ${project.version}<![CDATA[</a></strong></div>]]></header>
                         <bottom>Public domain {inceptionYear}, {currentYear}</bottom>
+                        <offlineLinks>
+                            <offlineLink>
+                                <url>https://stanio.github.io/xbrz-java/apidocs/</url>
+                                <location>${maven.multiModuleProjectDirectory}/docs/apidocs/</location>
+                            </offlineLink>
+                        </offlineLinks>
                         <additionalOptions>
                             <additionalOption>--add-reads</additionalOption>
                             <additionalOption>io.github.stanio.xbrz.tool=io.github.stanio.xbrz.awt</additionalOption>