(doc) Update references to PMD documentation

Follow-up to pmd/pmd@04b0859fb2
(pmd_releases/6.3.0-123-g04b0859fb2)

Author: pzygielo

src/main/resources/rulesets/java/maven-pmd-plugin-default.xml

@@ -29,7 +29,7 @@ under the License.
 
         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 @@ under the License.
     <rule ref="category/java/performance.xml/BigIntegerInstantiation" />
     <rule ref="category/java/performance.xml/BooleanInstantiation" />
 
-</ruleset>
+</ruleset>

src/main/resources/rulesets/maven.xml

@@ -26,7 +26,7 @@ under the License.
         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>

src/site/apt/examples/multi-module-config.apt.vm

@@ -122,7 +122,7 @@ whizbang
 +-----+
 
   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

src/site/apt/examples/usingRuleSets.apt.vm

@@ -45,7 +45,7 @@ Using Rule Sets
  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 @@ The default ruleset
 
     <rule ref="category/java/performance.xml/BigIntegerInstantiation" />
     <rule ref="category/java/performance.xml/BooleanInstantiation" />
-+-----+
++-----+