Improve examples, add defer, async, and pre- loading KaTeX (#1580)

* Wrap `katex.render` examples with try...catch

* Add starter template with defer in README.md

* Update documentation to use `defer` and `onload`

* Add documentation on async and pre loading KaTeX

* Use sha384 for SRI

* Revise wording according to comments

* Minor changes

* Fix article

docs/api.md

@@ -6,23 +6,38 @@ title: API
 Call `katex.render` with a TeX expression and a DOM element to render into:
 
 ```js
-katex.render("c = \\pm\\sqrt{a^2 + b^2}", element);
+katex.render("c = \\pm\\sqrt{a^2 + b^2}", element, {
+    throwOnError: false
+});
 ```
 
 To avoid escaping the backslash (double backslash), you can use
 [`String.raw`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/raw)
 (but beware that `${`, `\u` and `\x` may still need escaping):
 ```js
-katex.render(String.raw`c = \pm\sqrt{a^2 + b^2}`, element);
+katex.render(String.raw`c = \pm\sqrt{a^2 + b^2}`, element, {
+    throwOnError: false
+});
 ```
 
-If KaTeX can't parse the expression, it throws a `katex.ParseError` by default.
-See [handling errors](error.md) for configuring how to handle errors.
-
-## Server side rendering or rendering to a string
+## Server-side rendering or rendering to a string
 To generate HTML on the server or to generate an HTML string of the rendered math, you can use `katex.renderToString`:
 
 ```js
-var html = katex.renderToString("c = \\pm\\sqrt{a^2 + b^2}");
+var html = katex.renderToString("c = \\pm\\sqrt{a^2 + b^2}", {
+    throwOnError: false
+});
 // '<span class="katex">...</span>'
 ```
+
+## Handling errors
+
+The examples above use the `throwOnError: false` option, which renders invalid
+inputs as the TeX source code in red (by default), with the error message as
+hover text.  Without this option, invalid LaTeX will cause a
+`katex.ParseError` exception to be thrown.  See [handling errors](error.md).
+
+## Configuring KaTeX
+
+The last argument to `katex.render` and `katex.renderToString` can contain
+[a variety of rendering options](options.md).