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/node.md

@@ -0,0 +1,23 @@
+---
+id: node
+title: Node.js
+---
+## npm
+Install with `npm`:
+
+```bash
+npm install katex
+# or globally
+npm install -g katex
+```
+
+## Yarn
+Install with `Yarn`:
+
+```bash
+yarn add katex
+# or globally
+yarn global add katex
+```
+<br>
+> You can manually download the package from [here](https://github.com/Khan/KaTeX/releases)