Docs: invalid default `:files` value on `hex.publish` page

[Bartosz Błaszków]

The docs for task hex.publish contain an old default value for :files key in package configuration, i.e. .formatter.exs is missing. https://hexdocs.pm/hex/Mix.Tasks.Hex.Publish.html#module-package-configuration

The PR to fix that would be trivial, however, I'm thinking about a more "permanent" solution for keeping them in sync - WDYT about:

1. Moving the @default_files attribute inside the hex.build task before moduledoc
2. Referencing that attribute inside the moduledoc
3. Exposing it via a public function
4. Call that function inside a moduledoc of hex.publish

EDIT: I think it could be further extended to expose the whole paragraph on package configuration, as it is duplicated in both places

[Eric Meadows-Jönsson]

Great suggestion, a PR with those changes would be very appreciated.