Use dropdown to create projects

Refs https://gitlab.com/arkindex/requests/-/issues/645

We'll use a dropdown button with 2 choices to replace the current "New Project" button on the corpus list:

1. "Empty project", will redirect to the current form
2. "Import transkribus project", will redirect to the transkribus form

Also remove the "Transkribus" button from the user menu.

Finally, update the transkribus doc in a MR you'll refs here