Update documentation to use `cisagov/gh-skeleton`

# 🐛 Summary #

In some places this documentation still uses the old script that used to exist at `project_setup/scripts/skeleton` or includes instructions on how to set this up manually. The documentation should instead refer to the [cisagov/gh-skeleton](https://github.com/cisagov/gh-skeleton) plugin for the `gh` GitHub CLI, or possibly the `setup-env` script at the repository root.

Examples of files that need to be updated are `project_setup/skeletonize-existing-repository.md` and `project_setup/README.md`.