A document can be imported with JSON via the Import Document endpoint. To do so, set your file type to be x-application/vnd.lucid.standardImport, which is the Standard Import file type.
For more information on how to use the Standard Import, refer to the extensive reference documentation here.
For examples of how to use the Standard Import, refer to the /standard-import folder in Lucid's repository of Sample Lucid REST Applications. This repo also contains some example .lucid ZIP files you can reference or use in your own projects. These can be found here.
The POST Document Contents endpoint can only create a new document, not modify an existing document's contents. However, it can be used to create either a Lucidchart OR Lucidspark document.