Document the Role::Logger in order to support additional log engines.

Without documentation of this role, it's hard for a developer to implement
support for additional log engines.

Hooks are not documented yet because as of right now, there's no proof that
they are working as expected.
Fix the POD for Role::Logger.
Add POD for Logger::Console.
Fix link to the documentation of Dancer2:::Core::Role::Logger.
Remove empty synopsis header fom Logger::Diag.
Merge branch 'devel' into doc/logger
Merge branch 'doc/logger' into devel