There's no help document that I know, just a quick introduction the first time you open Stackedit in a new browser. However, for your question about Dropbox, I happen to know from using various apps with Dropbox, that if you want to publish to the root of Dropbox, just type "/". If you want to publish to a specific folder or subfolder on Dropbox, you would ...
It's possible but requires quite a bit of programming. StackEdit accepts custom extensions where you can add extra functionality to it.
See UserCustom extensions · benweet/stackedit Wiki
Custom scripts are added at the very end of Settings » Extensions:
Pass the cursor at the bottom right corner, to extend a toolbar that includes the ? button:
But there are several formats that are not included like spoilers and tables, so the best could be to try the examples on https://stackoverflow.com/documentation/markdown