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/sidebars.json

@@ -0,0 +1,8 @@
+{
+  "docs": {
+    "Installation": ["node", "browser"],
+    "Usage": ["api", "cli", "autorender", "libs"],
+    "Configuring KaTeX": ["options", "security", "error", "font"],
+    "Misc": ["supported", "issues"]
+  }
+}