Add overrides to fix print styles

Fixes flutter#2974

src/_sass/base/_print_overrides.scss

@@ -0,0 +1,38 @@
+// Overrides for printing so only page content is printed
+@media print {
+  // Ignore navigation elements and other non-necessary interactive ones
+  .site-header, .subnav, .site-footer, .site-sidebar, .navbar,
+  #site-toc--side, #page-github-links, #cookie-notice, .site-banner,
+  .code-excerpt__copy-btn, .breadcrumb {
+    display: none !important;
+  }
+
+  // Make sure content fills up 100% of width
+  .site-content {
+    max-width: 100% !important;
+    margin-left: 0;
+    padding-left: 0;
+    border: none;
+    flex: 1;
+  }
+
+  // Remove DartPad iframes since they are not functional
+  .code-excerpt iframe {
+    display: none;
+  }
+
+  // Display underlines under links
+  a {
+    text-decoration: underline;
+  }
+
+  // Remove external link icon and link path
+  a:after {
+    content: none !important;
+  }
+
+  // Show borders around notes and code blocks
+  .alert, pre {
+    border: 1px solid black;
+  }
+}

src/_sass/main.scss

@@ -53,3 +53,4 @@
 
 // -- Overrides
 @import 'base/site_overrides';
+@import 'base/print_overrides';