Disallow search engine indexing

The documentation is currently public, but we do not want it to appear
in public searches (yet).

.prettierignore

@@ -5,3 +5,4 @@ package-lock.json
 Pipfile.lock
 public/
 site/
+overrides/

docs/robots.txt

+User-agent: *
+Disallow: /

mkdocs.yml

@@ -30,6 +30,7 @@ theme:
     - search.highlight
     - search.share
     - search.suggest
+  custom_dir: overrides
 
 extra_css:
   - stylesheets/extra.css

overrides/main.html

+{# vim: ft=jinja-html #}
+{% extends "base.html" %}
+{% block extrahead %}
+  {{ super() }}
+  <meta name="robots" content="noindex,noarchive,nofollow" />
+{% endblock %}