"DataFusion User Guide" in ballista/rust/client/README.md is not found

[kou]

Describe the bug
https://github.com/apache/arrow-datafusion/blob/master/ballista/rust/client/README.md#starting-a-cluster has a link to "DataFusion User Guide" but it returns 404.

To Reproduce
Steps to reproduce the behavior:

Open https://github.com/apache/arrow-datafusion/tree/master/docs/user-guide .

Expected behavior
https://arrow.apache.org/datafusion/user-guide/introduction.html may be a good location rather than https://github.com/apache/arrow-datafusion/tree/master/docs/source/user-guide .

Additional context
#993 changed docs/ path.