docs: Mark external links with an icon

Helps to distinguish internal links in the documentation from links
leading to outside resources.

docs/conf.py

@@ -50,6 +50,7 @@ html_css_files = [
 html_theme_options = {
     'logo_only': True,
     'style_nav_header_background': "#000",
+    'style_external_links': True,
 }
 
 def setup(app):