Update some welcome links (#3902)

Summary:
Some old welcome links are 'Page Not Found'.

## Changelog

Some old links are 'Page Not Found', so update them as follows:
1. Update "Using Flipper" link to 'https://fbflipper.com/docs/features'
2. Update "Add Flipper Support to Your App" link to 'https://fbflipper.com/docs/getting-started'

Pull Request resolved: https://github.com/facebook/flipper/pull/3902

Test Plan: Pass the test on a Mac.

Reviewed By: lblasa

Differential Revision: D37927376

Pulled By: passy

fbshipit-source-id: 0aac129932395e3f2ce3afa8c47cc637f3e814a8

desktop/flipper-ui-core/src/sandy-chrome/WelcomeScreen.tsx

@@ -202,7 +202,7 @@ function WelcomeScreenContent() {
           icon={<RocketOutlined />}
           title="Using Flipper"
           subtitle="Learn how Flipper can help you debug your App"
-          onClick={openExternal('https://fbflipper.com/docs/features/index')}
+          onClick={openExternal('https://fbflipper.com/docs/features')}
         />
         <Row
           icon={<AppstoreAddOutlined />}
@@ -214,9 +214,7 @@ function WelcomeScreenContent() {
           icon={<CodeOutlined />}
           title="Add Flipper Support to Your App"
           subtitle="Get started with these pointers"
-          onClick={openExternal(
-            'https://fbflipper.com/docs/getting-started/index',
-          )}
+          onClick={openExternal('https://fbflipper.com/docs/getting-started')}
         />
         <Row
           icon={<BugOutlined />}