saveDocument abstract method
Saves a document to the specified Firestore collection.
collection
: The name of the Firestore collection.
data
: A Map containing the document data to be saved. If the data contains an 'id',
the document will be updated, otherwise, a new document will be created.
Returns a Future that resolves when the operation is complete.
Implementation
Future<void> saveDocument(String collection, Map<String, dynamic> data);