Add version warning banner for "latest" sphinx docs

docs/sphinx/source/_static/version-alert.js

@@ -0,0 +1,34 @@
+"use strict";
+
+// Source:
+// https://github.com/anymail/django-anymail/blob/4c443f5515d1d5269a95cb54cf75057c56a3b150/docs/_static/version-alert.js
+// via:
+// https://github.com/qucontrol/krotov/blob/969fc980346e6411903de854118c48c51208a810/docs/_static/version-alert.js
+// following instructions here:
+// https://michaelgoerz.net/notes/showing-a-warning-for-the-latest-documentation-on-readthedocs.html
+
+function warnOnLatestVersion() {
+
+  // The warning text and link is really specific to RTD hosting,
+  // so we can just check their global to determine version:
+  if (!window.READTHEDOCS_DATA || window.READTHEDOCS_DATA.version !== "latest") {
+    return;  // not latest, or not on RTD
+  }
+
+  var warning = document.createElement('div');
+  warning.setAttribute('class', 'admonition danger');
+  warning.innerHTML = "<p class='first admonition-title'>Note</p> " +
+    "<p class='last'> " +
+    "This document is for an <strong>unreleased development version</strong>. " +
+    "Documentation is available for the <a href='/en/stable/'>current stable release</a>, " +
+    "or for older versions through the &ldquo;v:&rdquo; menu at bottom left." +
+    "</p>";
+  warning.querySelector('a').href = window.location.pathname.replace('/latest', '/stable');
+
+  var parent = document.querySelector('div.body')
+    || document.querySelector('div.document')
+    || document.body;
+  parent.insertBefore(warning, parent.firstChild);
+}
+
+document.addEventListener('DOMContentLoaded', warnOnLatestVersion);

docs/sphinx/source/conf.py

@@ -223,6 +223,8 @@ def setup(app):
     # Override footnote callout CSS to be normal text instead of superscript
     # In-line links to references as numbers in brackets.
     app.add_css_file("reference_format.css")
+    # Add a warning banner at the top of the page if viewing the "latest" docs
+    app.add_javascript("version-alert.js")
 
 # -- Options for LaTeX output ---------------------------------------------