Add a warning banner when not reading the stable docs

docs/_templates/layout.html

@@ -0,0 +1,9 @@
+{% extends "!layout.html" %}
+{% block document %}
+{% if version != stable_version %}
+<div class="wy-alert info">
+    <p>You are currently browsing documentation for the <b>{{ version }}</b> branch. Documentation for the stable <b>{{ stable_version }}</b> branch can be found <a href="/{{ stable_version }}/">here</a>.</p>
+</div>
+{% endif %}
+{{ super() }}
+{% endblock %}

docs/conf.py

@@ -36,6 +36,7 @@ html_context = {
     'github_user': 'mailu',
     'github_repo': 'mailu',
     'github_version': version,
+    'stable_version': '1.5',
     'versions': [
         ('1.5', '/1.5/'),
         ('master', '/master/')