Add method to create a new document

If a user has just installed app, they would need a .txt file or similar to preexist somewhere.

We should add a method to create a new such document. Maybe we can make it default to `.txt`, to maximize the chances the user can view or edit with other mobile or cloud tools.

The issue names no files, tests, or entry points. Start by locating the existing document-opening and file-creation flow in the Dart/Flutter app, then determine where a new-document method belongs and how a default .txt document should be represented; done means a newly installed user can create and open a new text document without an existing file.

Written by the indexing model from the issue text.