Docs: Mark example "hcloud_token" variable as sensitive

Prevents the field's values from showing up in CLI output and in Terraform Cloud

Signed-off-by: Alex Andrews <alias-dev@protonmail.com>

website/docs/index.html.markdown

@@ -17,7 +17,9 @@ Use the navigation to the left to read about the available resources.
 ```hcl
 # Set the variable value in *.tfvars file
 # or using the -var="hcloud_token=..." CLI option
-variable "hcloud_token" {}
+variable "hcloud_token" {
+  sensitive = true # Requires terraform >= 0.14
+}
 
 # Configure the Hetzner Cloud Provider
 provider "hcloud" {