Fix broken links in generated groovydocs

### Issue description

### Issue description

Broken links can be identified and verified as fixed using https://github.com/lycheeverse/lychee/releases/.  

```
./gradlew grails-doc:build
cd \grails-doc\build\docs\
lychee-x86_64-windows.exe . --format markdown --output results.md --exclude ".*.css$" --exclude "^http://localhost"
```

review `results.md`