[Docs] add v0.15.0 to versions.json, remove pre-releases

Author: mbrookes

docs/src/www/versions.json

@@ -1,9 +1,6 @@
 [
   "HEAD",
-  "v0.15.0-beta.2",
-  "v0.15.0-beta.1",
-  "v0.15.0-alpha.2",
-  "v0.15.0-alpha.1",
+  "v0.15.0",
   "v0.14.4",
   "v0.14.3",
   "v0.14.2",
@@ -46,4 +43,4 @@
   "v0.3.2",
   "v0.3.1",
   "v0.3.0"
-]
+]