DOC: display alternative versions in the CookBook

Documentation/Cookbook/_static/html/versions.html

+<div class="rst-versions" data-toggle="rst-versions" role="note" aria-label="versions">
+    <span class="rst-current-version" data-toggle="rst-current-version">
+        <span class="fa fa-book"> Other Versions</span>
+        <span id="current-version-text"></span>
+        <span class="fa fa-caret-down"></span>
+    </span>
+    <div class="rst-other-versions">
+        <dl>
+            <dt>Versions</dt>
+            <dd><a href="https://www.orfeo-toolbox.org/CookBook-6.2/">6.2.0</a></dd>
+            <dd><a href="https://www.orfeo-toolbox.org/CookBook-6.4/">6.4.0</a></dd>
+            <dd><a href="https://www.orfeo-toolbox.org/CookBook-6.6.1/">6.6.1</a></dd>
+            <dd><a href="https://www.orfeo-toolbox.org/CookBook-develop/">develop</a></dd>
+        </dl>
+    </div>
+</div>

Documentation/Cookbook/_static/js/versions.js

+
+// This piece of javascript is used to display the versions widget at the lower
+// left corner of the html CookBook for OTB
+
+window.addEventListener('load', function() {
+    var request = new XMLHttpRequest();
+
+    request.onreadystatechange = function () {
+        if (request.readyState === XMLHttpRequest.DONE) {
+            if (request.status == 200) {
+
+                // Display the html div received with the AJAX request
+                document.body.insertAdjacentHTML('beforeend', request.responseText);
+
+                // Element showing version number in the header (made by sphinx)
+                var e = document.querySelector(".version");
+
+                // Element showing version number in the bottom left (the one we want)
+                var y = document.querySelector("#current-version-text");
+
+                if (e && y) {
+                    // Swap them
+                    y.innerHTML = "v: " + e.innerHTML;
+                    e.remove();
+                }
+
+            } else {
+                console.log("Request for versions.html failed with HTTP ", request.status);
+            }
+        }
+    };
+
+    var versions_div_url = "_static/html/versions.html";
+
+    // If we are hosted on OTB website, get the latest version of the widget from
+    // develop so that past releases can show links to future ones
+    if (window.location.hostname == "www.orfeo-toolbox.org") {
+        versions_div_url = "https://www.orfeo-toolbox.org/CookBook-develop/_static/html/versions.html";
+    }
+
+    request.open('GET', versions_div_url);
+    request.send();
+});

Documentation/Cookbook/rst/conf.py.in

@@ -22,9 +22,11 @@ import sphinx_rtd_theme
 #sys.path.insert(0, os.path.abspath('.'))
 
 # Customize read the docs theme a bit with a custom css
+# and a custom version selection widget
 # taken from https://stackoverflow.com/a/43186995/5815110
 def setup(app):
     app.add_stylesheet("css/otb_theme.css")
+    app.add_javascript("js/versions.js")
 
 # -- General configuration ------------------------------------------------