Add border-radius to button for consistency with flutter.dev (#11664)

In the top navbar, the 'Get Started' button has sharp angles. Instead, we should make it rounded by adding a border radius.

Fixes #11622 

---

Now looks better with round shape 😊

<img width="143" alt="image"
src="https://github.com/user-attachments/assets/497921b6-df79-47ba-bfb7-5a520b09cab5"
/>

src/_sass/components/_button.scss

@@ -30,13 +30,12 @@ a, button {
     width: fit-content;
     white-space: nowrap;
     outline-offset: 2px;
-
+    border-radius: 24px;
     font-weight: 500;
     padding: 0.5rem 1rem;
     background-color: $site-color-primary;
     color: $site-color-white;
     text-decoration: none;
-    border-radius: 0;
     cursor: pointer;
     user-select: none;