Add UpCloud plugin documentation (#11587)

* Add UpCloud plugin documentation

* Update UpCloud plugin repository path

* Add sourceBranch to UpCloud plugin description

* Alphabetically sort plugin

Co-authored-by: Wilken Rivera <wilken@hashicorp.com>

website/data/plugins-manifest.json

@@ -270,6 +270,14 @@
     "version": "latest",
     "pluginTier": "community"
   },
+  {
+    "title": "UpCloud",
+    "path": "upcloud",
+    "repo": "UpCloudLtd/packer-plugin-upcloud",
+    "version": "latest",
+    "pluginTier": "community",
+    "sourceBranch": "master"
+  },
   {
     "title": "Vagrant",
     "path": "vagrant",