Apply micronaut-platform BOM to forge documentation configuration

Fixes the `Build Grails Forge` CI jobs that 0143553 broke. The
`documentation` configuration is registered by the buildsrc.groovydoc
plugin and is independent of compileClasspath / testRuntimeClasspath, so
the micronaut-platform BOM applied in each forge module's main
`dependencies { }` block does not reach it. After 0143553 dropped
the explicit `:$groovyVersion` from the documentation entries, the
groovy-* artifacts had no version source and groovydoc failed with
`Could not find org.apache.groovy:groovy-templates:.`

Apply the same BOM to the documentation configuration here so the
unversioned coordinates resolve through groovy-bom, matching the
approach used in the other forge configurations.

Verified: `./gradlew :grails-forge-cli:groovydoc :grails-forge-api:groovydoc
:grails-forge-core:groovydoc` succeeds.

Assisted-by: claude-code:claude-opus-4-7

Author: jamesfredley

grails-forge/gradle/doc-config.gradle

@@ -24,6 +24,11 @@ groovydocEnhancer {
 }
 
 dependencies {
+    // The `documentation` configuration is registered by the buildsrc.groovydoc
+    // plugin and is independent of compileClasspath/testRuntimeClasspath, so the
+    // micronaut-platform BOM we apply elsewhere does not reach it. Apply it here so
+    // the unversioned groovy-* artifacts below resolve through groovy-bom.
+    documentation platform("io.micronaut.platform:micronaut-platform:$micronautVersion")
     documentation 'org.apache.groovy:groovy-templates'
     documentation 'org.apache.groovy:groovy-dateutil'
     documentation 'org.apache.groovy:groovy-ant'