refactor(docs-infra): move common `.group-buttons` styles to `_buttons.scss` (#40704)

Both `<aio-contributor-list>` and `<aio-resource-list>` contain
`.group-buttons` elements. Previously, global styles for
`.group-buttons` were defined in `_contributor.scss` (i.e. the styles
from `_contributor.scss` were also applied to `.group-buttons` in
`<aio-resource-list>`). Also, `_contributor.scss` and `_resources.scss`
defined some duplicate styles for `.group-buttons`.

This commit moves the `.group-buttons` styles to `_buttons.scss`, which
contains common button-related styles.

PR Close #40704

aio/src/styles/2-modules/_buttons.scss

@@ -98,6 +98,23 @@ a.filter-button {
   }
 }
 
+.group-buttons {
+  margin: 32px auto;
+
+  a.selected {
+    background-color: $blue;
+    color: $white;
+  }
+
+  @media (max-width: 480px) {
+    .filter-button.button {
+      font-size: 1.2rem;
+      padding: 0;
+      margin: 0;
+    }
+  }
+}
+
 [mat-button], [mat-raised-button], [mat-button], [mat-raised-button] {
   text-transform: uppercase;
 }

aio/src/styles/2-modules/_contributor.scss

@@ -16,25 +16,6 @@ aio-contributor-list {
   }
 }
 
-.group-buttons {
-  margin: 32px auto;
-
-  @media (max-width: 480px) {
-    .filter-button.button {
-      font-size: 1.2rem;
-      padding: 0;
-      margin: 0;
-    }
-  }
-
-  a {
-    &.selected {
-      background-color: $blue;
-      color: $white;
-    }
-  }
-}
-
 .contributor-group {
   display: flex;
   flex-direction: row;

aio/src/styles/2-modules/_resources.scss

@@ -11,17 +11,6 @@
 
 .resources-container {
   position: relative;
-
-  .group-buttons {
-
-    @media (max-width: 480px) {
-      .button {
-        font-size: 1.2rem;
-        padding: 0;
-        margin: 0;
-      }
-    }
-  }
 }
 
 .grid-fixed:after, .grid-fixed:before {