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/error.md

@@ -0,0 +1,17 @@
+---
+id: error
+title: Handling Errors
+---
+If KaTeX encounters an error (invalid or unsupported LaTeX) and `throwOnError`
+hasn't been set to `false`, then it will throw an exception of type
+`ParseError`.  The message in this error includes some of the LaTeX
+source code, so needs to be escaped if you want to render it to HTML.
+
+In particular, you should convert `&`, `<`, `>` characters to
+`&amp;`, `&lt;`, `&gt;` (e.g., using `_.escape`)
+before including either LaTeX source code or exception messages in your
+HTML/DOM.  (Failure to escape in this way makes a `<script>` injection
+attack possible if your LaTeX source is untrusted.)
+
+Alternatively, you can set `throwOnError` to `false` to use built-in behavior
+of rendering the LaTeX source code with hover text stating the error.