Ensure the project is good for docs.rs

[gabotechs]

We should make sure that this project has a good documentation page in docs.rs, right now we have this:

https://docs.rs/datafusion-distributed/latest/datafusion_distributed/index.html

I think we are missing something other things, like a better landing page, and probably a bit of better structure overall.

Another topic is formatting inside docstrings, for examples, should we be using backticks for every referenced symbol?