Update/shiro docs

### What is this PR for?
Currently, Zeppelin has two authentication docs. One is  [**Authentication**](https://zeppelin.incubator.apache.org/docs/0.6.0-incubating-SNAPSHOT/security/authentication.html) and the other is [**Shiro Authentication**](https://zeppelin.incubator.apache.org/docs/0.6.0-incubating-SNAPSHOT/manual/shiroauthentication.html). As a user, it's little bit confused. So I changed the category of `shiroauthentication.md` file from `manual` to `security` and also changed the name of `Authentication` to `Authentication for NGINX`. Please see the below screenshot images :)

### What type of PR is it?
Improvement

### Todos
* [x] - Add `conf/shiro.ini` file to `.gitignore`
* [x] - Update `zeppelin-login.png` screenshot image file in `shiroauthentication.md`
* [x] - Change the category of `shiroauthentication.md` file from `manual` -> `security`
* [x] - Change `Authentication` -> `Authentication for NGINX`

### What is the Jira issue?

### How should this be tested?

### Screenshots (if appropriate)
1. Updating  zeppelin-login.png
    - before
![zeppelin-login 1](https://cloud.githubusercontent.com/assets/10060731/15422239/7ad7cf0c-1eb2-11e6-9106-70d5833d99af.png)

    - after
<img width="1272" alt="zeppelin-login" src="https://cloud.githubusercontent.com/assets/10060731/15422244/812541e6-1eb2-11e6-89bc-ed635bc9aaa8.png">

2. Changing the category of **Shiro Authentication** & Changing **Authentication** -> **Authentication for NGINX**
    - before
![screen shot 2016-05-20 at 5 31 34 pm](https://cloud.githubusercontent.com/assets/10060731/15422261/9c63cd92-1eb2-11e6-938d-959ff5ebbb0d.png)

 - after
![screen shot 2016-05-20 at 5 31 19 pm](https://cloud.githubusercontent.com/assets/10060731/15422270/a4a06d6c-1eb2-11e6-90ff-5ad18cb2ac61.png)

### Questions:
* Does the licenses files need update? No
* Is there breaking changes for older versions? No
* Does this needs documentation? No

Author: AhyoungRyu <[email protected]>

Closes apache#907 from AhyoungRyu/update/shiroDocs and squashes the following commits:

95d37ab [AhyoungRyu] Change title Authentication -> Authentication for NGINX
d290937 [AhyoungRyu] Change Shiro Authentication docs category
749ce8c [AhyoungRyu] Change shiroauthentication.md location
20b8ae9 [AhyoungRyu] Update login screenshot image in shiroauthentication.md
122afc2 [AhyoungRyu] Add conf/shiro.ini file to .gitignore

.gitignore

@@ -25,6 +25,7 @@ conf/keystore
 conf/truststore
 conf/interpreter.json
 conf/notebook-authorization.json
+conf/shiro.ini
 
 # other generated files
 spark/dependency-reduced-pom.xml

docs/_includes/themes/zeppelin/_navigation.html

@@ -94,17 +94,15 @@
                 <li role="separator" class="divider"></li>
                 <!-- li><span><b>Security</b><span></li -->
                 <li><a href="{{BASE_PATH}}/security/overview.html">Security Overview</a></li>
-                <li><a href="{{BASE_PATH}}/security/authentication.html">Authentication</a></li>
+                <li><a href="{{BASE_PATH}}/security/authentication.html">Authentication for NGINX</a></li>
+                <li><a href="{{BASE_PATH}}/security/shiroauthentication.html">Shiro Authentication</a></li>
                 <li><a href="{{BASE_PATH}}/security/notebook_authorization.html">Notebook Authorization</a></li>
                 <li><a href="{{BASE_PATH}}/security/interpreter_authorization.html">Interpreter Authorization</a></li>
                 <li role="separator" class="divider"></li>
                 <!-- li><span><b>Development</b><span></li -->
                 <li><a href="{{BASE_PATH}}/development/writingzeppelininterpreter.html">Writing Zeppelin Interpreter</a></li>
                 <li><a href="{{BASE_PATH}}/development/howtocontribute.html">How to contribute (code)</a></li>
                 <li><a href="{{BASE_PATH}}/development/howtocontributewebsite.html">How to contribute (website)</a></li>
-                <li role="separator" class="divider"></li>
-                <!-- li><span><b>Shiro Security</b><span></li -->
-                <li><a href="{{BASE_PATH}}/manual/shiroauthentication.html">Shiro Authentication</a></li>
               </ul>
             </li>
           </ul>

docs/security/authentication.md

@@ -1,7 +1,7 @@
 ---
 layout: page
-title: "Authentication"
-description: "Authentication"
+title: "Authentication for NGINX"
+description: "Authentication for NGINX"
 group: security
 ---
 <!--
@@ -17,7 +17,7 @@ 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.
 -->
-# Authentication
+# Authentication for NGINX
 
 Authentication is company-specific.
 

docs/manual/shiroauthentication.md → docs/security/shiroauthentication.md

@@ -2,7 +2,7 @@
 layout: page
 title: "Shiro Security for Apache Zeppelin"
 description: ""
-group: manual
+group: security
 ---
 <!--
 Licensed under the Apache License, Version 2.0 (the "License");
@@ -59,7 +59,7 @@ Then you can browse Zeppelin at [http://localhost:8080](http://localhost:8080).
 ####4. Login
 Finally, you can login using one of the below **username/password** combinations.
 
-<center><img src="../assets/themes/zeppelin/img/docs-img/zeppelin-login.png" width="40%" height="40%"></center>
+<center><img src="../assets/themes/zeppelin/img/docs-img/zeppelin-login.png"></center>
 
 ```
 admin = password1