refactor templates layout

@see https://gohugo.io/templates/new-templatesystem-overview/

layouts/_partials/footer/text.html

@@ -0,0 +1,40 @@
+{{- /*
+  Returns the site footer text.
+
+  @params licences_data Site data of licences, indexed by SPDX IDs.
+  @params licence   Licence identifier.
+  @params licence_link  Manually-defined licence URL. Optional.
+  */
+-}}
+
+<!-- Validation -->
+
+{{- if ( not ( isset . "author_name" ) ) -}}
+    {{- errorf "No site author defined" -}}
+{{- end -}}
+
+{{- if ( not ( isset . "report_issue_url" ) ) -}}
+    {{- errorf "No 'Report Issue' URL defined" -}}
+{{- end -}}
+
+{{- if ( not ( isset . "privacy_policy_url" ) ) -}}
+    {{- errorf "No 'Privacy Policy' URL defined" -}}
+{{- end -}}
+
+{{- if ( not ( isset . "sitemap_path" ) ) -}}
+    {{- errorf "No sitemap path defined" -}}
+{{- end -}}
+
+{{- if ( not ( isset . "last_build_time" ) ) -}}
+    {{- errorf "No last build time defined" -}}
+{{- end -}}
+
+<!-- Rendering -->
+
+<p>
+    By <a href="/">{{- .author_name -}}</a> |
+    <a href="{{- .report_issue_url -}}">Report an Issue</a> |
+    <a href="{{- .privacy_policy_url -}}">Privacy Policy</a> | 
+    <a href="{{- .sitemap_path -}}">Sitemap</a> |
+    Built: {{ .last_build_time -}}
+</p>