Migrate docs to antora

- First step of migration to antora by creating a structure
  which can be build locally.
- From docs to spring-security-kerberos-docs
- Gradle build handles needed docs logic, etc
- Relates #166

spring-security-kerberos-docs/modules/ROOT/examples/DummyUserDetailsService.java

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2015 the original author or authors.
+ *
+ * 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 org.springframework.security.kerberos.docs;
+
+import org.springframework.security.core.authority.AuthorityUtils;
+import org.springframework.security.core.userdetails.User;
+import org.springframework.security.core.userdetails.UserDetails;
+import org.springframework.security.core.userdetails.UserDetailsService;
+import org.springframework.security.core.userdetails.UsernameNotFoundException;
+
+//tag::snippetA[]
+public class DummyUserDetailsService implements UserDetailsService {
+
+    @Override
+    public UserDetails loadUserByUsername(String username)
+            throws UsernameNotFoundException {
+        return new User(username, "notUsed", true, true, true, true,
+                AuthorityUtils.createAuthorityList("ROLE_USER"));
+    }
+
+}
+//end::snippetA[]