Build/nit: cache output of generatedMarkdownDocs (#2967)

`(Java)Exec` tasks are not cacheable by default, as annotated with `@DisableCachingByDefault`.
Adding an `outputs.cacheIf { true }` enables caching on those tasks.

Author: snazy

tools/config-docs/site/build.gradle.kts

@@ -52,6 +52,7 @@ val generatedMarkdownDocs = tasks.register<JavaExec>("generatedMarkdownDocs") {
 
   mainClass = "org.apache.polaris.docs.generator.ReferenceConfigDocsGenerator"
 
+  outputs.cacheIf { true }
   outputs.dir(generatedMarkdownDocsDir)
   inputs.files(doclet)
   inputs.files(genProjects)