Add architecture to docs

website/content/docs/boxes/box_repository.mdx

@@ -50,7 +50,9 @@ It is a JSON document, structured in the following way:
           "name": "virtualbox",
           "url": "http://example.com/bionic64_010_virtualbox.box",
           "checksum_type": "sha1",
-          "checksum": "foo"
+          "checksum": "foo",
+          "architecture": "amd64",
+          "default_architecture": true
         }
       ]
     }
@@ -59,7 +61,8 @@ It is a JSON document, structured in the following way:
 ```
 
 As you can see, the JSON document can describe multiple versions of a box,
-multiple providers, and can add/remove providers in different versions.
+multiple providers, and can add/remove providers/architectures in different
+versions.
 
 This JSON file can be passed directly to `vagrant box add` from the
 local filesystem using a file path or via a URL, and Vagrant will

website/content/docs/vagrantfile/machine_settings.mdx

@@ -42,6 +42,13 @@ the name of the synced folder plugin.
   of an installed box or a shorthand name of a box in
   [HashiCorp's Vagrant Cloud](/vagrant/vagrant-cloud).
 
+- `config.vm.box_architecture` (string) - The architecture of the box to be used.
+  Supported architecture values: "386", "amd64", "arm", "arm64", "ppc64le", "ppc64",
+  "mips64le", "mips64", "mipsle", "mips", and "s390x". The special value `:auto` will
+  detect the host architecture and fetch the appropriate box, if available. When the
+  value is set to `nil`, it will fetch the box flagged as the default architecture.
+  Defaults to `:auto`.
+
 - `config.vm.box_check_update` (boolean) - If true, Vagrant will check for updates to
   the configured box on every `vagrant up`. If an update is found, Vagrant
   will tell the user. By default this is true. Updates will only be checked