Improve doc's main image loading time

Doc's main image is currently heavy.
This PR makes it 5x smaller while keeping equivalent design

Pairs: @gerhard, @slumbering

Signed-off-by: guillaume <guillaume.derouville@gmail.com>

docs/introduction/what_is.md

@@ -12,7 +12,7 @@ Dagger is a portable devkit for CICD. It helps you develop powerful CICD pipelin
 * Avoid CI lock-in. Dagger runs natively on Github Actions, Gitlab, CircleCI, Jenkins, AWS Codebuild, Tekton, and more.
 * Test and debug your pipelines locally; catch configuration errors in seconds with static type checking.
 
-![Dagger_Website_Ship](https://user-images.githubusercontent.com/216487/122216381-328a3500-ce61-11eb-907f-d2b6f66b3b10.png)
+<img src="/img/what-is-dagger.png" alt="Dagger_Website_Ship" style={{maxWidth: '50%'}} />
 
 ## Dagger is alpha software
 

website/src/css/custom.scss

@@ -152,14 +152,9 @@ code {
   }
 
   img {
-    max-width: 380px;
     display: block;
     margin: auto;
     width: 100%;
-
-    @include desktopWide {
-      max-width: 500px;
-    }
   }
 }