Landing page mobile responsiveness fix (#5362)

Summary:
This Improves the Mobile Responsiveness of the  Site

## Changelog

UI - Responsiveness Improved

Pull Request resolved: #5362

Test Plan:
### Before :

![image](https://github.com/facebook/flipper/assets/98690069/7f24e742-101f-47f7-8ec6-3904d8fc1d1e)
![image](https://github.com/facebook/flipper/assets/98690069/3379f68c-fe83-45a0-8fd3-168e77341cfb)

### After :

![image](https://github.com/facebook/flipper/assets/98690069/031de8ef-9175-4167-82e6-1fb9a42ab3f1)
![image](https://github.com/facebook/flipper/assets/98690069/d93073ad-268d-4ead-b3b3-5b4806e263c3)

Reviewed By: mweststrate

Differential Revision: D52259121

Pulled By: passy

fbshipit-source-id: e2dcc9dbd058fe95ad3ce53b186e5709a8d52365

website/static/css/custom.css

@@ -570,4 +570,18 @@ div[class^="announcementBarContent"] a:hover {
     line-height: 22px;
     padding: 6px 30px;
   }
+  .splash {
+    min-height: 750px;
+  }
+
+  .splash h2 {
+    margin-bottom: 40px;
+  }
+
+  .splash .landing-btn {
+    margin-top: 15px;
+  }
+  a.landing-btn {
+    margin-top: 10px;
+  }
 }