Add configurable keyboard shortcut to focus search input (#1411)

Adds a keyboard shortcut to use Ctrl/Cmd and a configurable key to focus the search input. The key is configurable with `search.focus_shortcut_key`; enables it on the core docs site with `'k'` (default on many sites).

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Co-authored-by: Matt Wang <mxw@cs.washington.edu>

assets/js/just-the-docs.js

@@ -145,6 +145,18 @@ function searchLoaded(index, docs) {
   var currentInput;
   var currentSearchIndex = 0;
 
+  {%- if site.search.focus_shortcut_key %}
+  // add event listener on ctrl + <focus_shortcut_key> for showing the search input
+  jtd.addEvent(document, 'keydown', function (e) {
+    if ((e.ctrlKey || e.metaKey) && e.key === '{{ site.search.focus_shortcut_key }}') {
+      e.preventDefault();
+
+      mainHeader.classList.add('nav-open');
+      searchInput.focus();
+    }
+  });
+  {%- endif %}
+
   function showSearch() {
     document.documentElement.classList.add('search-active');
   }
@@ -488,7 +500,7 @@ jtd.setTheme = function(theme) {
 
 function navLink() {
   var pathname = document.location.pathname;
-  
+
   var navLink = document.getElementById('site-nav').querySelector('a[href="' + pathname + '"]');
   if (navLink) {
     return navLink;