Add option to limit export type

[twindscheif]

Describe the feature you'd like

Right now you have multiple export options if export is enabled for the users'role.
I would like to have an option to limit the export options.
Use case:
I like to export a branded PDF, but i do not want to allow a full export via new "Portable ZIP"-Option.

At least i would like to have an option in the .env-file with a e. g. comma separated list like
EXPORT_OPTIONS=PDF,HTML,MRKDWN,ZIP,TXT

Later a fine grained Permission Subrole which includes the permssion to export as ZIP would be awesome.

Describe the benefits this would bring to existing BookStack users

You may want to share your Book for guest users just as a branded PDF and not as a raw importable ZIP-Archive.
On the other hand - for loggedin users - you may like to allow the export of the book in a importable format (like ZIP)

Can the goal of this request already be achieved via other means?

No

Have you searched for an existing open/closed issue?

• I have searched for existing issues and none cover my fundamental request

How long have you been using BookStack?

1 to 5 years

Additional context

No response