(doc) Update references to PMD documentation

Follow-up to https://github.com/pmd/pmd/commit/04b0859fb2
(pmd_releases/6.3.0-123-g04b0859fb2)
diff --git a/src/main/resources/rulesets/java/maven-pmd-plugin-default.xml b/src/main/resources/rulesets/java/maven-pmd-plugin-default.xml
index 86bf7a2..472cade 100644
--- a/src/main/resources/rulesets/java/maven-pmd-plugin-default.xml
+++ b/src/main/resources/rulesets/java/maven-pmd-plugin-default.xml
@@ -29,7 +29,7 @@
 
         This ruleset might be used as a starting point for an own customized ruleset [0].
 
-        [0] https://pmd.github.io/latest/pmd_userdocs_understanding_rulesets.html
+        [0] https://pmd.github.io/latest/pmd_userdocs_making_rulesets.html
     </description>
 
     <rule ref="category/java/bestpractices.xml/AvoidUsingHardCodedIP" />
@@ -90,4 +90,4 @@
     <rule ref="category/java/performance.xml/BigIntegerInstantiation" />
     <rule ref="category/java/performance.xml/BooleanInstantiation" />
 
-</ruleset>
\ No newline at end of file
+</ruleset>
diff --git a/src/main/resources/rulesets/maven.xml b/src/main/resources/rulesets/maven.xml
index 706d8ee..e37ca6a 100644
--- a/src/main/resources/rulesets/maven.xml
+++ b/src/main/resources/rulesets/maven.xml
@@ -26,7 +26,7 @@
         Customized PMD ruleset for Maven, see [0] for more information.
         This ruleset is references by org.apache.maven:maven-parent.
 
-        [0] https://pmd.github.io/latest/pmd_userdocs_understanding_rulesets.html
+        [0] https://pmd.github.io/latest/pmd_userdocs_making_rulesets.html
     -->
 
     <description>
diff --git a/src/site/apt/examples/multi-module-config.apt.vm b/src/site/apt/examples/multi-module-config.apt.vm
index 4856e16..1ab1a89 100644
--- a/src/site/apt/examples/multi-module-config.apt.vm
+++ b/src/site/apt/examples/multi-module-config.apt.vm
@@ -122,7 +122,7 @@
 +-----+
 
   The ruleset references some built-in rules. For more information about rulesets, see
-  {{{https://pmd.github.io/pmd-${pmdVersion}/pmd_userdocs_understanding_rulesets.html}Understanding Rulesets}}
+  {{{https://pmd.github.io/pmd-${pmdVersion}/pmd_userdocs_making_rulesets.html}Making Rulesets}}
   and {{{./usingRuleSets.html}Using Rule Sets}}.
 
 * Configure the other projects to use it
diff --git a/src/site/apt/examples/usingRuleSets.apt.vm b/src/site/apt/examples/usingRuleSets.apt.vm
index 7ac4e5f..7aba4cb 100644
--- a/src/site/apt/examples/usingRuleSets.apt.vm
+++ b/src/site/apt/examples/usingRuleSets.apt.vm
@@ -45,7 +45,7 @@
  with the PMD tool, you do not need to specificy the absolute path of the file. It will be resolved by
  the plugin. But if the rule set is a custom rule set, you need to specify its absolute path.
  
- See {{{https://pmd.github.io/pmd-${pmdVersion}/pmd_userdocs_understanding_rulesets.html}Understanding Rulesets}}
+ See {{{https://pmd.github.io/pmd-${pmdVersion}/pmd_userdocs_making_rulesets.html}Making Rulesets}}
  if you want to create your own custom ruleset.
 
  <<Note:>> Starting with PMD 5.0 and Maven PMD Plugin 3.0 the paths to the
@@ -151,4 +151,4 @@
 
     <rule ref="category/java/performance.xml/BigIntegerInstantiation" />
     <rule ref="category/java/performance.xml/BooleanInstantiation" />
-+-----+
\ No newline at end of file
++-----+