Update all README and CONTRIBUTING files to point to the install selector

[vyasr]

To reduce duplication, we should have all RAPIDS repo README, CONTRIBUTING, and other associated files point to the install selector and remove any other instructions, at least for released packages. The selector also supports nightlies so that is covered. The README files should point to the CONTRIBUTING (or an equivalent) for instructions on building from source so that the instructions for building from source can also be single-sourced.