Move Anvil to the top level per the Proposed Structure doc.

content/Anvil/_index.md

content/Anvil/adding_ssh_key_pairs.md

content/Anvil/anvil_instance_types.md

content/Anvil/available_images.md

content/Anvil/connecting_to_linux_instances_from_mac.md

content/Anvil/connecting_to_linux_instances_from_windows.md

content/Anvil/connecting_to_linux_instances_using_x2go.md

content/Anvil/connecting_to_the_anvil_vpn.md

content/Anvil/connecting_to_windows_instances.md

content/Anvil/creating_an_instance.md

content/Anvil/creating_and_attaching_a_volume.md

content/Anvil/creating_ssh_key_pairs_on_mac.md

content/Anvil/creating_ssh_key_pairs_on_windows.md

content/Anvil/formatting_and_mounting_a_volume_in_linux.md

content/Anvil/formatting_and_mounting_a_volume_in_windows.md

content/Anvil/resizing_an_instance.md

content/Anvil/what_are_the_per_group_resource_limits.md