mkdocs: header source code link open in new browser tab

Add source.html partial override file to open GitHub repository link in header
with a new browser tab/window

Author: practicalli-johnny

overrides/partials/source.html

@@ -0,0 +1,9 @@
+<a href="{{ config.repo_url }}" title="{{ lang.t('source') }}" class="md-source" data-md-component="source" target="_blank">
+    <div class="md-source__icon md-icon">
+        {% set icon = config.theme.icon.repo or "fontawesome/brands/git-alt" %}
+        {% include ".icons/" ~ icon ~ ".svg" %}
+    </div>
+    <div class="md-source__repository">
+        {{ config.repo_name }}
+    </div>
+</a>