added the version list for docs on master (#1309)

Author: alirana01

docs/_static/docs_version.js

@@ -0,0 +1,9 @@
+var DOCUMENTATION_VERSIONS = {
+    DEFAULTS: { has_targets: false,
+                supported_targets: [ "esp32" ]
+              },
+    VERSIONS: [
+        { name: "latest" },
+        { name: "release-v4.0.0", pre_release: true }
+    ]
+};