Add dynamic version information to docs (#275)

* Add dynamic version list to docs

Author: grst

docs/index.rst

@@ -8,4 +8,6 @@
    api
    ir-biology
    glossary
+   versions
    zzz_bibliography
+

docs/versions.rst

@@ -0,0 +1,21 @@
+Other versions
+==============
+
+There are other versions of the documentation available: 
+
+
+.. raw:: html
+
+   <ul id="versions_container">
+	<li><a href="https://icbi-lab.github.io/scirpy">latest release</a></li>
+	<li><a href="https://icbi-lab.github.io/scirpy/develop">latest development version</a></li>
+   </ul>
+   <script type="text/javascript">
+   	fetch("https://icbi-lab.github.io/scirpy/versions/versions.json")
+	   .then(response => response.json())
+	   .then(versions => versions.forEach((x, i) => {
+	   	document.getElementById("versions_container").innerHTML += '<li><a href="https://icbi-lab.github.io/scirpy/tags/' + x + '/">' + x + '</a></li>\n'
+           })).catch((error) => {
+	       document.getElementById("versions_container").innerHTML = "Could not download version information..."
+	   })
+   </script>