[FEATURE] Clear description of building own dataset

[ArtyomZemlyak]

Hello!
Thanks for your project!

I think that there is a need to add an extended description of creating your own dataset. Without references to other sources, since there, too, the description leaves much to be desired.
Or the simplest complete example from scratch.

The need is, since I see the main users cases just on the use of autogl on their own graphs. And for people who don’t really want to understand the intricacies of creating datasets of other libraries (maybe this wasn’t a big problem if there were normal examples in the documentation of these libraries).

Thanks!

[ArtyomZemlyak]

And have "ERROR" msg on trying load:
http://mn.cs.tsinghua.edu.cn/autogl/documentation

So I had to read this documentation:
https://autogl.readthedocs.io/en/latest/docfile/tutorial/t_dataset.html