website/commands/init: Add blurb on plugin selection
website/commands/init: Add blurb on plugin selection

This change adds text around how plugin selection for plugins defined in
a require_plugins block work. It also adds a small call out to users
that third party plugins are not verified by HashiCorp.

rnaveiras/packer,rnaveiras/packer,hashicorp/packer,hashicorp/packer,ricardclau/packer,ricardclau/packer,rnaveiras/packer,rnaveiras/packer,pecigonzalo/packer,ricardclau/packer,pecigonzalo/packer,rnaveiras/packer,ricardclau/packer,rnaveiras/packer,hashicorp/packer,hashicorp/packer,mitchellh/packer,mitchellh/packer,ricardclau/packer,pecigonzalo/packer,mitchellh/packer,mitchellh/packer,mitchellh/packer,pecigonzalo/packer,ricardclau/packer