Improve UX & documentation

Reviewed and manually tested MR !16. Clear improvements, the move to /admin/content makes sense.

Some minor findings:

• allowed_extensions default belongs in config, not just the form. The default is set only on the form display:
  '#default_value' => $config->get('allowed_extensions') ?: 'pdf doc docx txt',. If an admin never opens the settings form, path_file.settings:allowed_extensions stays empty. An empty file_extensions means no extension restriction (any file type uploadable). Add a config/install/path_file.settings.yml with allowed_extensions: 'pdf doc docx txt'.
• README still use references to 'entity/entities'
• info.yml description says "Creates a new content type which can will render a file at the given path.". You can remove the 'will'. How about: "Lets editors upload files and serve them at a given path."
• maybe out of scope, but the file extentions now do a regex, but Drupal normally uses a validator ('#element_validate' => [['Drupal\file\Plugin\Field\FieldType\FileItem', 'validateExtensions']]). Reusing it also allows . and _ (e.g.
  tar.gz), comma separators

Reviewed and tested locally; tests are green, also with the current 2.1.x merged. The mentioned problems are solved.

One left over occurrence in the UI of 'entity':

Otherwise all good.