Make GOV.UK Frontend assets paths work with http prefix

Co-authored-by: Oliver Byford <oliver.byford@digital.cabinet-office.gov.uk>

Author: lfdebrux

lib/assets/stylesheets/_govuk_tech_docs.scss

@@ -1,4 +1,15 @@
-$govuk-assets-path: "/assets/govuk/assets/" !default;
+@function tech-docs-image-url-function($filename) {
+  @return image-url($govuk-images-path + $filename)
+}
+
+@function tech-docs-font-url-function($filename) {
+  @return font-url($govuk-fonts-path + $filename)
+}
+
+$govuk-font-url-function: 'tech-docs-font-url-function';
+$govuk-image-url-function: 'tech-docs-image-url-function';
+
+$govuk-assets-path: "/assets/govuk/assets/";
 
 // Include only the bits of GOV.UK Frontend we need
 $govuk-new-link-styles: true;