Update instructions for project setup

# 💡 Summary #

Run through the current [project setup documentation](https://github.com/cisagov/development-guide/tree/develop/project_setup) and make sure it is up-to-date with current expectations and practices.

## Motivation and context ##

The current instructions for project setup still reference the [`skeleton`](https://github.com/cisagov/development-guide/blob/develop/project_setup/scripts/skeleton) script in this repository. We have since migrated to using the [`gh` CLI] and the [cisagov/gh-skeleton] extension. There may be other out-dated our inaccurate items as well.

[`gh` CLI]: https://cli.github.com/
[cisagov/gh-skeleton]: https://github.com/cisagov/gh-skeleton