Mapping create plugin tutorial to correct external URL

Summary: Previously "Create your own plugin" in welcome screen and "Create Plugins" in menu were directing to a URL https://fbflipper.com/docs/create-plugin.html which doesn't exist anymore. So pointing the URL to page for plugin creation tutorial https://fbflipper.com/docs/tutorial/intro.html

Reviewed By: jknoxville

Differential Revision: D15362962

fbshipit-source-id: 82a9dedb21911607f20884c9bb36e58d622e8431

src/MenuBar.js

@@ -350,7 +350,9 @@ function getTemplate(
         {
           label: 'Create plugins',
           click: function() {
-            shell.openExternal('https://fbflipper.com/docs/create-plugin.html');
+            shell.openExternal(
+              'https://fbflipper.com/docs/tutorial/intro.html',
+            );
           },
         },
         {

src/chrome/WelcomeScreen.js

@@ -146,7 +146,7 @@ export default class WelcomeScreen extends PureComponent<Props, State> {
           <Item
             onClick={() =>
               shell.openExternal(
-                'https://fbflipper.com/docs/create-plugin.html',
+                'https://fbflipper.com/docs/tutorial/intro.html',
               )
             }>
             <Icon size={20} name="magic-wand" color={brandColors.Flipper} />