GenerateDocs.java

package org.openrewrite.java;

import org.openrewrite.ExecutionContext;
import org.openrewrite.Recipe;
import org.openrewrite.SourceFile;
import org.openrewrite.Tree;
import org.openrewrite.internal.ListUtils;
import org.openrewrite.java.tree.J;
import org.openrewrite.marker.Markers;
import org.openrewrite.text.PlainText;

import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

public class GenerateDocs extends Recipe {

    @Override
    public String getDisplayName() {
        return "Generic docs";
    }

    @Override
    public String getDescription() {
        return "Generate docs based on some things.";
    }

    @Override
    protected List<SourceFile> visit(List<SourceFile> before, ExecutionContext ctx) {
        List<String> classNames = new ArrayList<>();

        for (SourceFile sourceFile : before) {
            new JavaIsoVisitor<Integer>() {
                @Override
                public J.ClassDeclaration visitClassDeclaration(J.ClassDeclaration classDecl, Integer integer) {
                    classNames.add(classDecl.getType().getFullyQualifiedName());
                    return super.visitClassDeclaration(classDecl, integer);
                }
            }.visit(sourceFile, 0);
        }

        PlainText doc = new PlainText(Tree.randomId(), Paths.get("docs/class-list.asciidoc"), Markers.EMPTY,
                "= A list of classes" + classNames.stream().map(name -> "* " + name).collect(Collectors.joining("\n", "\n", "")));

        return ListUtils.concat(before, doc);
    }
}

Relativization of the doc path relative to the project root will be handled for you.