[docs] provide means to have collapsible code blocks without adding a new theme

docs/_static/custom_collapsible_code.css

@@ -0,0 +1,17 @@
+.toggle {
+    padding-bottom: 1em ;
+}
+
+.toggle .header {
+    display: block;
+    clear: both;
+    cursor: pointer;
+}
+
+.toggle .header:after {
+    content: " ▼";
+}
+
+.toggle .header.open:after {
+    content: " ▲";
+}

docs/_static/custom_collapsible_code.js

@@ -0,0 +1,10 @@
+let makeCodeBlocksCollapsible = function() {
+    $(".toggle > *").hide();
+    $(".toggle .header").show();
+    $(".toggle .header").click(function() {
+        $(this).parent().children().not(".header").toggle({"duration": 400});
+        $(this).parent().children(".header").toggleClass("open");
+    });
+};
+// we could use the }(); way if we would have access to jQuery in HEAD, i.e. we would need to force the theme
+// to load jQuery before our custom scripts