arch_release_promotion/config.py:
Change `Settings.PRIVATE_TOKEN` to become an optional attribute, as it
is not required for readonly scenarios.
Order all `Settings` attributes alphabetically and add documentation for
them and their respective default values.
Change `Settings.validate_private_token()` to reflect the change in type
for `Settings.PRIVATE_TOKEN`.

tests/test_config.py:
Extend tests to also check for cases where `Settings.PRIVATE_TOKEN` is
None.