DOC: add run_async example to the docs

Currently we have a detailed description of the manual workflow of running an async query, here: https://pyvo.readthedocs.io/en/latest/dal/index.html#synchronous-vs-asynchronous-query

But it would be beneficial to start with an example of `run_async()` at the top, and then go into the weeds, rather than weeds first and then a side remark that `run_async()` does all of the above automatically. 