Learn more points to getting started

Summary:
`/docs/understand.html` doesn't exist anymore. Also "learn more" in the landing page pointed to `/docs/understand.html` which was broken. Pointed "Learn More" to getting started doc page.

`/docs/understand.html` explained about rsocket which will be little overwhelming for the user to be navigated to right from the landing page.

Also removed the links pointing to  `/docs/understand.html`

Reviewed By: passy

Differential Revision: D15147954

fbshipit-source-id: cbd67d7f234c438507f24db946cd463e8827cb78

src/chrome/WelcomeScreen.js

@@ -131,7 +131,9 @@ export default class WelcomeScreen extends PureComponent<Props, State> {
           </Version>
           <Item
             onClick={() =>
-              shell.openExternal('https://fbflipper.com/docs/understand.html')
+              shell.openExternal(
+                'https://fbflipper.com/docs/getting-started.html',
+              )
             }>
             <Icon size={20} name="rocket" color={brandColors.Flipper} />
             <FlexColumn>

website/core/Footer.js

@@ -35,7 +35,6 @@ class Footer extends React.Component {
           <div>
             <h5>Docs</h5>
             <a href="/docs/getting-started.html">Getting Started</a>
-            <a href="/docs/understand.html">Understanding Flipper</a>
             <a href="/docs/js-setup.html">Creating Plugins</a>
           </div>
           <div>

website/pages/en/index.js

@@ -26,7 +26,7 @@ class Index extends React.Component {
                 href="https://www.facebook.com/fbflipper/public/mac">
                 Download
               </a>
-              <a className="btn" href="/docs/understand.html">
+              <a className="btn" href="/docs/getting-started.html">
                 Learn more
               </a>
             </div>
@@ -70,7 +70,7 @@ class Index extends React.Component {
               data back and forth, calling functions, and listening for events
               on the mobile app.
             </p>
-            <a className="learnmore" href="/docs/understand.html">
+            <a className="learnmore" href="/docs/getting-started.html">
               Learn more
             </a>
           </div>