Update Portal styles & templates

Officially renames it the "XRP Ledger Developer Portal" and
adapts the Ripple Client Portal templates to the XRP Dev Portal:

- Table-of-Contents-like landing page
- Accordion-style side nav except on landing page
- Breadcrumbs except on landing page
- floating (right-side) in-page nav on doc pages
- Adds "Edit on GitHub" link
- Slight modifications to API tool styles

assets/js/jump-to-top.js

@@ -0,0 +1,15 @@
+$(document).ready(function() {
+  var TO_TOP_MIN = 50;
+  var TO_TOP_SPEED = 500;
+  var TO_TOP_POS = 0;
+  $(window).scroll(function () {
+    if ($(this).scrollTop() > TO_TOP_MIN) {
+       $('.jump-to-top').fadeIn();
+    } else {
+       $('.jump-to-top').fadeOut();
+    }
+  });
+  $(".jump-to-top").click(function() {
+    $("body").animate({scrollTop: TO_TOP_POS}, TO_TOP_SPEED)
+  });
+});