src/_includes/base.html

{% assign split_url = page.url | split:'/' %}
{% if split_url.size > 0 %}
  {% assign splitted_last = split_url | last | split:'.' %}
  {% if splitted_last.size > 1 %}
    {% assign depth = split_url | size | minus: 2 %}
  {% else %}
    {% assign depth = split_url | size | minus: 1 %}
  {% endif %}
  {% capture base %}./{% for i in (1..depth) %}../{% endfor %}{% endcapture %}
{% else %}
  {% capture base %}./{% endcapture %}
{% endif %}