docs: Improve helium doc

docs/development/helium/overview.md

@@ -25,3 +25,15 @@ limitations under the License.
 
 ## What is Helium? 
 
+Helium is a plugin system that can extend Zeppelin a lot. 
+For example, you can write [custom display system](./writing_spell.html) or 
+install already published one in [Heliun Online Registry](http://zeppelin.apache.org/helium_packages.html). 
+
+Currently, Helium supports 4 types of package.
+
+- [Helium Visualization](./writing_visualization_basic.html): Adding a new chart type
+- [Helium Spell](./writing_spell.html): Adding new interpreter, display system running on browser
+- [Helium Application](./writing_application.html) 
+- [Helium Interpreter](../writing_zeppelin_interpreter.html): Adding a new custom interpreter
+
+

docs/development/helium/writing_visualization_basic.md

@@ -205,3 +205,8 @@ You can browse localhost:9000. Everytime refresh your browser, Zeppelin will reb
 
 Once it's done, publish your visualization package using `npm publish`.
 That's it. With in an hour, your visualization will be available in Zeppelin's helium menu.
+
+### See More
+
+Check [Helium Visualization: Transformation](./writing_visualization_transformation.html) for more complex examples.
+

zeppelin-web/src/app/helium/helium.controller.js

@@ -177,9 +177,9 @@ export default function HeliumCtrl ($scope, $rootScope, $sce,
 
   const getHeliumTypeText = function (type) {
     if (type === HeliumType.VISUALIZATION) {
-      return `<a target="_blank" href="https://zeppelin.apache.org/docs/${$rootScope.zeppelinVersion}/development/writingzeppelinvisualization.html">${type}</a>` // eslint-disable-line max-len
+      return `<a target="_blank" href="https://zeppelin.apache.org/docs/${$rootScope.zeppelinVersion}/development/helium/writing_nvisualization.html">${type}</a>` // eslint-disable-line max-len
     } else if (type === HeliumType.SPELL) {
-      return `<a target="_blank" href="https://zeppelin.apache.org/docs/${$rootScope.zeppelinVersion}/development/writingzeppelinspell.html">${type}</a>` // eslint-disable-line max-len
+      return `<a target="_blank" href="https://zeppelin.apache.org/docs/${$rootScope.zeppelinVersion}/development/helium/writing_spell.html">${type}</a>` // eslint-disable-line max-len
     } else {
       return type
     }