Move the documentation package to its own subproject (#722)

This makes it easier to later migrate the package to Java 11. If we move
and migrate in a single PR, because of the portion of the contents that
s changed, git will have trouble recognizing that some files are
renamed *and* modified and treat them as distinct files, making code
review difficult.

docs/src/test/java/google/registry/documentation/FlowDocumentationTest.java

@@ -0,0 +1,56 @@
+// Copyright 2017 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package google.registry.documentation;
+
+import static com.google.common.truth.Truth.assertWithMessage;
+import static google.registry.util.BuildPathUtils.getProjectRoot;
+import static java.nio.charset.StandardCharsets.UTF_8;
+
+import com.google.common.base.Joiner;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import org.junit.jupiter.api.Test;
+
+/** Tests to ensure that generated flow documentation matches the expected documentation. */
+class FlowDocumentationTest {
+  private static final Path GOLDEN_MARKDOWN_FILEPATH = getProjectRoot().resolve("docs/flows.md");
+
+  private static final String UPDATE_COMMAND = "./gradlew :docs:flowDocsTool";
+  private static final String UPDATE_INSTRUCTIONS =
+      Joiner.on('\n')
+          .join(
+              "",
+              "-----------------------------------------------------------------------------------",
+              "Your changes affect the flow API documentation output. To update the golden version "
+                  + "of the documentation, run:",
+              UPDATE_COMMAND,
+              "");
+
+  @Test
+  void testGeneratedMatchesGolden() throws IOException {
+    // Read the markdown file.
+    Path goldenMarkdownPath =
+        GOLDEN_MARKDOWN_FILEPATH;
+
+    String goldenMarkdown = new String(Files.readAllBytes(goldenMarkdownPath), UTF_8);
+
+    // Don't use Truth's isEqualTo() because the output is huge and unreadable for large files.
+    DocumentationGenerator generator = new DocumentationGenerator();
+    if (!generator.generateMarkdown().equals(goldenMarkdown)) {
+      assertWithMessage(UPDATE_INSTRUCTIONS).fail();
+    }
+  }
+}