Load assets using a relative path

Migrated issue

Reported by: codebymikey

Related to !394 (merged)

Problem/Motivation

Currently the image assets used in the markdown files fail to render when viewed from the Gitlab repository.

This is because they currently use a /gitlab_templates/ prefix in order to render properly on GitLab pages.

Proposed resolution

We should use an appropriate variable so that Gitlab pages renders the relative asset paths properly.

Following the documentation on https://www.mkdocs.org/user-guide/configuration/#site_url, we should add the following to the mkdocs.yml file.

<?php
site_url: 'https://project.pages.drupalcode.org/gitlab_templates/'
?>

And that'll render the assets properly.

Related issue: Issue #3426311