KaTeX website & documentation (#1484)

* website/docs: initial commit

* Change secondaryColor

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

* Fix stylesheet link

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* Change documentation link to API(Usage)

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* Add `Libraries` in usage

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

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* Use KaTeX in the parent directory to build Markdown

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* Revise function support page. Avoid error msgs.

* General edit to function support page

docs/api.md

@@ -0,0 +1,28 @@
+---
+id: api
+title: API
+---
+## In-browser rendering
+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);
+```
+
+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);
+```
+
+If KaTeX can't parse the expression, it throws a `ParseError`. See [handling errors](error.md)
+for configuring how to handle errors.
+
+## 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}");
+// '<span class="katex">...</span>'
+```