Refactor author configuration system

[caendesilva]

Having a whole file for defining authors and requiring having a YAML parser added more complexity than benefits, see #442.

Instead of defining author shortcuts in the authors.yml, we define them in the config/hyde.php file using the new helper:

'authors' => [
  Author::create(
    username: 'mr_hyde', // Required username
    display_name: 'Mr. Hyde', // Optional display name
    website: 'https://hydephp.com' // Optional website URL
  ),
  Author::create('john_doe', 'John Doe')
]

Fixes #446