I'd like to be able to attach receipts, notes, and other documentation to a contribution. Is this possible?
In a word, yes. You'll have to implement this as custom field sets, which is easy. That's menu option Administer/Customize Data and Screens/Custom fields. You can have a custom type of 'file' as a custom field, which then allows you to attach a file. You can have as many of these fields as you require, and you can set the custom field set as applying to contributions only.