ci: change base image from Debian to Ubuntu 18.04 (#428)

Change base docker image from `node:x.x.x-buster-slim` to `ubuntu:18.04`
The image size reduces from 1.05GB to 327MB

- Install Node.js via nodesource https://github.com/nodesource/distributions
- Install Git via ppa:git-core/ppa https://git-scm.com/download/linux

.nojekyll

@@ -0,0 +1 @@
+This file makes sure that Github Pages doesn't process mdBook's output.

ayu-highlight.css

@@ -0,0 +1,79 @@
+/*
+Based off of the Ayu theme
+Original by Dempfi (https://github.com/dempfi/ayu)
+*/
+
+.hljs {
+  display: block;
+  overflow-x: auto;
+  background: #191f26;
+  color: #e6e1cf;
+  padding: 0.5em;
+}
+
+.hljs-comment,
+.hljs-quote {
+  color: #5c6773;
+  font-style: italic;
+}
+
+.hljs-variable,
+.hljs-template-variable,
+.hljs-attribute,
+.hljs-attr,
+.hljs-regexp,
+.hljs-link,
+.hljs-selector-id,
+.hljs-selector-class {
+  color: #ff7733;
+}
+
+.hljs-number,
+.hljs-meta,
+.hljs-builtin-name,
+.hljs-literal,
+.hljs-type,
+.hljs-params {
+  color: #ffee99;
+}
+
+.hljs-string,
+.hljs-bullet {
+  color: #b8cc52;
+}
+
+.hljs-title,
+.hljs-built_in,
+.hljs-section {
+  color: #ffb454;
+}
+
+.hljs-keyword,
+.hljs-selector-tag,
+.hljs-symbol {
+  color: #ff7733;
+}
+
+.hljs-name {
+    color: #36a3d9;
+}
+
+.hljs-tag {
+    color: #00568d;
+}
+
+.hljs-emphasis {
+  font-style: italic;
+}
+
+.hljs-strong {
+  font-weight: bold;
+}
+
+.hljs-addition {
+  color: #91b362;
+}
+
+.hljs-deletion {
+  color: #d96c75;
+}