Added Button if the user is logged in and css

Added a button if the user is already logged in which will directly go to the study
- make buttons minimal width

frontend/src/components/HeroSection.css

@@ -30,6 +30,7 @@
   display: flex;
   flex-direction: column;
   flex-wrap: wrap;
+  min-width: 60%;
 }
 
 .hero-btns .btn {

frontend/src/components/HeroSection.js

@@ -13,7 +13,14 @@ function HeroSection() {
       <h1>Herzlich Willkommen</h1>
       <div className="hero-btns">
         {isLoggedIn ? (
-          console.log("Logged in")
+          <Button
+            className="btns"
+            buttonStyle="btn--primary"
+            buttonSize="btn--large"
+            newTo="behavior"
+          >
+            Zur Studie
+          </Button>
         ) : (
           <>
             <Button