Add docs about sidebar property

[frozzare]

Example:

papi_property( [
  'type'     => 'sidebar',
  'title'    => 'Sidebar test',
  'slug'     => 'sidebar_test',
  'settings' => [
    'placeholder' => 'Select sidebar',
    'render'      => false
  ]
] )

Render setting:

• True value will render the sidebar using dynamic_sidebar and return the html.
• False value will disable the rendering and return the sidebar id.

Source:

• wp-papi/papi@fcf1bee