Fix broken links inside app

Summary:
Searched for broken link patterns inside the app itself. I think I found them all but it's not certain.

Patterns searched for and replaced:
```
.html
getting-started/)
getting-started)
getting-started"
```

and also searched the repo for regex `\]\(.*)` and checked them by eye.

Reviewed By: passy

Differential Revision: D21306944

fbshipit-source-id: a2e09b0fd8677f5f26e5cc4a06805b474247f7e6

desktop/app/src/MenuBar.tsx

@@ -365,15 +365,15 @@ function getTemplate(
         {
           label: 'Getting started',
           click: function () {
-            shell.openExternal('https://fbflipper.com/docs/getting-started/');
+            shell.openExternal(
+              'https://fbflipper.com/docs/getting-started/index',
+            );
           },
         },
         {
           label: 'Create plugins',
           click: function () {
-            shell.openExternal(
-              'https://fbflipper.com/docs/tutorial/intro.html',
-            );
+            shell.openExternal('https://fbflipper.com/docs/tutorial/intro');
           },
         },
         {