Add banner saying documentation is work in progress

mkdocs.yml

@@ -137,6 +137,7 @@ theme:
   font:
     text: Roboto
     code: Roboto Mono
+  custom_dir: overrides
 markdown_extensions:
   - admonition
   - def_list

overrides/main.html

@@ -0,0 +1,14 @@
+{% extends "base.html" %}
+
+{% block announce %}
+
+<p>
+Work in progress.
+</p>
+
+<p>
+The information found here is up to date but incomplete. Many pages are still empty or missing.
+Please refer to <a href="https://docs.cscs.ch">docs.cscs.ch</a> for documentation missing from this site for the time being.
+</p>
+
+{% endblock %}