create-next-app --example-path should support a local path for private repos

[glpecile]

Goals

1. Add the ability to support local paths using the --example-path parameter in create-next-app
2. Add better documentation for --example-path

Non-Goals

No response

Background

It would be nice to have a way to create new apps from a template that is private and stored locally, without the need of having it be a github repo.

At first I thought with the --example-path parameter I could do this and thought it was a bug or something wrong on my end. As discussed here it would be nice to implement something like this in the feature.

Proposal

Expand the existing --example-path to copy from the local directory instead of fetching it from a repo when there's no example added.

[nerdyman]

I also spent a bit of time running in circles with this assuming --example-path worked for local templates. Would be great if local templates were supported so we can bootstrap app from our own private templates. We have a custom template our monorepo at work so the GitHub URL approach won't work for us.