Update packaging guide and repo-review to match spec13

[Carreau]

See https://github.com/scientific-python/specs/pull/324

[Carreau]

Thanks for the review.

Let's wait on the agreement on the spec and I'll update this.

[henryiii]

I know there are not many tests yet, but there is a testing system set up if you want to add a couple of tests.

[Carreau]

closing for now to reduce noise in opened PRs.