Incorporate doc/aws-batch.md into new documentation system

[tsibley]

The doc/aws-batch.md file contains most of the information on setting up the --aws-batch runner, but it is not currently integrated into the new Sphinx/RTD doc system. It should be split into at least two separate pages which focus on a) using --aws-batch and b) initial setup.

Additionally, there is undocumented information on --detach usage which should be extracted from the changelog and commit messages and --help output and made proper parts of the documentation.