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/AuthProviderConfig.java

@@ -0,0 +1,69 @@
+/*
+ * 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.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.security.config.annotation.authentication.builders.AuthenticationManagerBuilder;
+import org.springframework.security.config.annotation.web.builders.HttpSecurity;
+import org.springframework.security.config.annotation.web.configuration.WebSecurityConfigurerAdapter;
+import org.springframework.security.config.annotation.web.servlet.configuration.EnableWebMvcSecurity;
+import org.springframework.security.kerberos.authentication.KerberosAuthenticationProvider;
+import org.springframework.security.kerberos.authentication.sun.SunJaasKerberosClient;
+
+//tag::snippetA[]
+@Configuration
+@EnableWebMvcSecurity
+public class AuthProviderConfig extends WebSecurityConfigurerAdapter {
+
+    @Override
+    protected void configure(HttpSecurity http) throws Exception {
+        http
+            .authorizeRequests()
+                .antMatchers("/", "/home").permitAll()
+                .anyRequest().authenticated()
+                .and()
+            .formLogin()
+                .loginPage("/login").permitAll()
+                .and()
+            .logout()
+                .permitAll();
+    }
+
+    @Override
+    protected void configure(AuthenticationManagerBuilder auth) throws Exception {
+        auth
+            .authenticationProvider(kerberosAuthenticationProvider());
+    }
+
+    @Bean
+    public KerberosAuthenticationProvider kerberosAuthenticationProvider() {
+        KerberosAuthenticationProvider provider =
+        		new KerberosAuthenticationProvider();
+        SunJaasKerberosClient client = new SunJaasKerberosClient();
+        client.setDebug(true);
+        provider.setKerberosClient(client);
+        provider.setUserDetailsService(dummyUserDetailsService());
+        return provider;
+    }
+
+    @Bean
+    public DummyUserDetailsService dummyUserDetailsService() {
+        return new DummyUserDetailsService();
+    }
+
+}
+//end::snippetA[]