Custom JS is available as an add-on to the pro plan.
Custom JS allows you to add custom executable code globally. It is the equivalent of adding a <script> tag with JS code into every page.

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Adding Custom JavaScript

Any .js file inside the content directory of your docs will be included in every documentation page. For example, you can add the following ga.js file to enable Google Analytics across the entire documentation. 
window.dataLayer = window.dataLayer || [];
function gtag() {
  dataLayer.push(arguments);
}
gtag('js', new Date());

gtag('config', 'TAG_ID');
Please use with caution to not introduce security vulnerabilities.