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/build.gradle

@@ -0,0 +1,44 @@
+// Copyright 2020 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.
+
+dependencies {
+  def deps = rootProject.dependencyMap
+
+  compile deps['com.beust:jcommander']
+  compile deps['com.google.code.findbugs:jsr305']
+  compile deps['com.google.flogger:flogger']
+  compile deps['com.google.guava:guava']
+  compile deps['com.google.re2j:re2j']
+  compile project(':core')
+  compile project(':util')
+
+  testCompile deps['com.google.truth:truth']
+  testCompile deps['com.thoughtworks.qdox:qdox']
+  testCompile deps['org.junit.jupiter:junit-jupiter-api']
+  testCompile deps['org.junit.jupiter:junit-jupiter-engine']
+}
+
+task flowDocsTool(type: JavaExec) {
+  systemProperty 'test.projectRoot', rootProject.projectRootDir
+  classpath = sourceSets.main.runtimeClasspath
+  main = 'google.registry.documentation.FlowDocumentationTool'
+
+  def arguments = []
+  if (rootProject.flowDocsFile) {
+    arguments << "--output_file=${rootProject.flowDocsFile}"
+  } else {
+    arguments << "--output_file=${rootProject.projectRootDir}/docs/flows.md"
+  }
+  args arguments
+}