Add mention that sidebar fields are not displayed on the index view

[adrianthedev]

Docs page: https://docs.avohq.io/3.0/resource-sidebar.html
Customer asked: https://discord.com/channels/740892036978442260/1282674626039779430/1372522781224206336

[Paul-Bob]

Also mention the helper to register fields on both sidebar and index without repeating the definition:

def fields_bag(**extra_args)
  field :name ..., **extra_args
  field :skill ..., **extra_args
end

def fields
  fields_bag(only_on: :index)

  sidebar do
    fields_bag
  end
end