Add syntax highlighting to improve code usability

[deniak]

Describe the issue
Some of our pages list a lot of commands and code but with the current style, it can be difficult to visualize these.
For instance, if we use code from markdown, the following markup will be generated:

<p>simple command</p>

<p><code class="language-plaintext highlighter-rouge">echo "Hello World"</code></p>

<p>JS code</p>

<div class="language-javascript highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">function</span> <span class="nx">sayHello</span><span class="p">(</span><span class="nx">name</span><span class="p">)</span> <span class="p">{</span>
  <span class="k">if</span> <span class="p">(</span><span class="o">!</span><span class="nx">name</span><span class="p">)</span> <span class="p">{</span>
    <span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="dl">'</span><span class="s1">Hello World</span><span class="dl">'</span><span class="p">);</span>
  <span class="p">}</span> <span class="k">else</span> <span class="p">{</span>
    <span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="s2">`Hello </span><span class="p">${</span><span class="nx">name</span><span class="p">}</span><span class="s2">`</span><span class="p">);</span>
  <span class="p">}</span>
<span class="p">}</span>
</code></pre></div></div>

<p>another type of commands: <code class="language-plaintext highlighter-rouge">foo bar</code></p>

Screenshot of the result

Using syntax highlighting and added a background color to code, samp and kbd would greatly improve the visibility, e.g.

Given that pages containing code will likely be generated from markdown files using jekyll, it may make sense to use one of the jekyll syntax highlight styles. FYI, I used monokai.sublime from https://github.com/mzlogin/rouge-themes/tree/master/dist to generate the above.