The website in pmd 7 has a few more pages (since #3162) which should be fleshed out.

https://docs.pmd-code.org/pmd-doc-7.0.0-SNAPSHOT/pmd_languages_java.html
https://docs.pmd-code.org/pmd-doc-7.0.0-SNAPSHOT/pmd_languages_apex.html

They would ideally include:

• Some doc about the language-specific features and APIs
  • Apex
    • ApexMultifileAnalysis
      • see [apex] Apexlink POC #2830
  • Java
    • eg [java] New type resolution #2689
    • see also [doc] Document TypeRes API and Symbols API #4319
  • Visualforce
    • eg [vf] Provide expression type information to Visualforce rules to avoid false positives #2864
    • note: This has actually been implemented for PMD 6.30.0
• Violation Decorators (see [core] Violation decorators #3902, [core] Implement violation decorators #4050)
• Exhaustive documentation of the available language properties ([core] Language properties #2518), or other configuration things, like the expected structure of the project
  • link to language specific properties (language_properties.md)
• Some high-level overview of how the module is implemented,
  • eg what semantic analysis we do
  • eg for apex, how we use apex jorje
  • eg for java, how we use ASM