fix(css): footer positioning on large screens #418

Adds flexbox layout to ensure footer stays at bottom of viewport
regardless of content height. Fixes footer floating mid-page on
pages like 404.

Author: alarcritty

static/css/main.css

@@ -606,3 +606,24 @@ iframe.poll {
     border:  1px solid var(--docsearch-primary-color)
   }
 }
+
+/* Force footer to bottom of viewport. */
+html {
+  height: 100%;
+}
+body {
+  min-height: 100vh;
+  display: flex;
+  flex-direction: column;
+  margin: 0;
+}
+body > nav {
+  flex-shrink: 0;
+}
+body > main {
+  flex: 1 0 auto;
+}
+body > footer {
+  flex-shrink: 0;
+  margin-top: auto;
+}