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/core/Footer.js

@@ -0,0 +1,96 @@
+/**
+ * Copyright (c) 2017-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+const React = require('react');
+
+class Footer extends React.Component {
+  docUrl(doc, language) {
+    const baseUrl = this.props.config.baseUrl;
+    return baseUrl + 'docs/' + (language ? language + '/' : '') + doc;
+  }
+
+  pageUrl(doc, language) {
+    const baseUrl = this.props.config.baseUrl;
+    return baseUrl + (language ? language + '/' : '') + doc;
+  }
+
+  render() {
+    const currentYear = new Date().getFullYear();
+    return (
+      <footer className="nav-footer" id="footer">
+        <section className="sitemap">
+          <a href={this.props.config.baseUrl} className="nav-home">
+            {this.props.config.footerIcon && (
+              <img
+                src={this.props.config.baseUrl + this.props.config.footerIcon}
+                alt={this.props.config.title}
+                width="66"
+                height="58"
+              />
+            )}
+          </a>
+          <div>
+            <h5>Docs</h5>
+            <a href={this.docUrl('node.html', this.props.language)}>
+              Installation
+            </a>
+            <a href={this.docUrl('api.html', this.props.language)}>
+              Usage
+            </a>
+            <a href={this.docUrl('options.html', this.props.language)}>
+              Configuration
+            </a>
+            <a href={this.docUrl('support.html', this.props.language)}>
+              Misc
+            </a>
+          </div>
+          <div>
+            <h5>Community</h5>
+            <a href={this.pageUrl('users.html', this.props.language)}>
+              Who is using KaTeX?
+            </a>
+            <a href="https://gitter.im/Khan/KaTeX">Gitter Chat</a>
+            <a
+              href="http://stackoverflow.com/questions/tagged/katex"
+              target="_blank"
+              rel="noreferrer noopener">
+              Stack Overflow
+            </a>
+          </div>
+          <div>
+            <h5>More</h5>
+            <a
+              className="github-button"
+              href={this.props.config.repoUrl}
+              data-icon="octicon-star"
+              data-count-href="/Khan/KaTeX/stargazers"
+              data-show-count={true}
+              data-count-aria-label="# stargazers on GitHub"
+              aria-label="Star this project on GitHub">
+              Star
+            </a>
+          </div>
+        </section>
+
+        <a
+          href="https://www.khanacademy.org/"
+          target="_blank"
+          className="fbOpenSource">
+          <img
+            src={this.props.config.baseUrl + 'img/khan-academy.png'}
+            alt="Khan Academy"
+            width="180"
+            height="17"
+          />
+        </a>
+        <section className="copyright">{this.props.config.copyright}</section>
+      </footer>
+    );
+  }
+}
+
+module.exports = Footer;