Make header docs dropdown fully visible at all widths (#250)

From viewport widths less than ~1300px down to the breakpoint at 1024px
when the TOC is removed and accessed via the burger icon the Docs
drop-down menu is partly off-screen because of the increased width.

Aligning dropdowns to the left prevents part of the docs dropdown from
disappearing off the left of the viewport.

src/css/header.css

@@ -406,7 +406,7 @@ body {
     border-radius: 0 0 0.25rem 0.25rem;
     display: none;
     top: 100%;
-    right: 0;
+    left: 0;
     min-width: 100%;
     position: absolute;
   }