Consolidate README.md and main documentation site

[alamb]

Thanks to #5500 and @martin-g, https://arrow.apache.org/datafusion/ is now built directly from the source

Currently the README https://github.com/apache/arrow-datafusion#readme contains partially duplicative information because it needed to be the most up to date

This ticket tracks consolidating the two sources of information:

1. Move all content from README to https://github.com/apache/arrow-datafusion/tree/main/docs
2. Leave README with a pointer to the main docs