[docs] troubleshooting guide
Hi all! This is a PR adding a troubleshooting guide for Datasets docs. 
I went through the library's GitHub Issues and Forum questions and identified a few issues that are common enough that I think it would be valuable to include them in the troubleshooting guide. These are: 
- creating a dataset from a folder and not following the required format
- authentication issues when using `push_to_hub`
- `Too Many Requests` with `push_to_hub`
- Pickling issues when using Dataset.from_generator()

There's also a section on asking for help. Please let me know if there are other common issues or advice that we can include here. 