Use relative paths in tutorials and examples

[thvasilo]

Our tutorials like https://graphstorm.readthedocs.io/en/v0.3.1/tutorials/quick-start.html are using absolute paths, assuming they are being executed from within the GS Docker container.

When referring to files in the repository we should be using paths relative to the root repo directory, to improve UX and allow users to directly copy-paste code from the guides.