KaTeX website & documentation (#1484)

* website/docs: initial commit

* Change secondaryColor

* Fix index.css not being copied and included on global stylesheet

* Fix stylesheet link

[skip ci]

* Change documentation link to API(Usage)

[skip ci]

* Add `Libraries` in usage

[skip ci]

* Remove documentation from `README.md` and add link to the site

[skip ci]

* Use KaTeX in the parent directory to build Markdown

[skip ci]

* Revise function support page. Avoid error msgs.

* General edit to function support page

website/static/js/index.js

@@ -0,0 +1,32 @@
+window.startup = function() {
+    var tex = document.getElementsByClassName("tex");
+    Array.prototype.forEach.call(tex, function(el) {
+        katex.render(el.getAttribute("data-expr"), el);
+    });
+
+    var demoInput = document.getElementById("demo-input");
+    var demoOutput = document.getElementById("demo-output");
+
+    function doDemo() {
+        try {
+            katex.render(demoInput.value, demoOutput, {
+                displayMode: true
+            });
+        } catch(err) {
+            while(demoOutput.lastChild) {
+                demoOutput.removeChild(demoOutput.lastChild);
+            }
+            var msg = document.createTextNode(err.message);
+            var span = document.createElement("span");
+            span.appendChild(msg);
+            demoOutput.appendChild(span);
+            span.setAttribute("class", "errorMessage");
+        }
+    }
+
+    demoInput.addEventListener("input", function() {
+        doDemo();
+    });
+
+    doDemo();
+};