feat: add configurable help text to list pages

Resolves #103

people/templates/people/organisation/list.html

@@ -14,6 +14,14 @@
     <a class="btn btn-success"
         href="{% url 'people:organisation.create' %}">New Organisation</a>
 
+    {% with config.ORGANISATION_LIST_HELP as help_text %}
+        {% if help_text %}
+            <div class="alert alert-info mt-3 pb-0">
+                {{ help_text|linebreaks }}
+            </div>
+        {% endif %}
+    {% endwith %}
+
     <table class="table table-borderless">
         <tbody>
             {% for country, organisations in orgs_by_country.items %}