Feature Request: Allow placeholder text for guestbook questions (single and multiline)

Overview of the Feature Request

Allow a user setting up a guestbook to add placeholder text for single and multi line question input fields.

What kind of user is the feature intended for?
(Example users roles: API User, Curator, Depositor, Guest, Superuser, Sysadmin)

Any user with permissions to create a guestbook.

What inspired the request?

ADA would like to provide more information and/or examples to the users as to how they should respond to a free-form question.

What existing behavior do you want changed?

Allow placeholder text that will appear in the guestbook answer form fields.

Any open or closed issues related to this feature request?
Couldn't find any.

Are you thinking about creating a pull request for this feature?
No resources to do to.

The issue names no files, tests, or entry points, so first locate guestbook question configuration and the answer form handling single-line and multiline fields. Done means a guestbook creator can set placeholder text for both question types and users see it in the corresponding answer fields; add coverage for both cases.