Clean Up: Remove non-maintained repos

[tpike3]

Projectmesa has three repos that are not maintained.

1. Mesa Schelling Exmaple - last update 7 years ago
2. Project Mesa Planning - last update 8 years ago
3. Presentations - last update 8 years ago.

Recommend we archive to avoid confusion with users and simplify projectmesa (#1624 )

I can do it but want concurrence of another maintainer (@jackiekazil , @rht )

[jackiekazil]

RE: 2 - I deleted.
RE: 1 - I think that the original paper or something else points to that -- I archived the repo, but I am hesitant to delete. We can add a note as well.
RE: 3 - I always liked the idea of a central location for this for people to find an discover other resources. I didn't want to put this in the docs, b/c the act of having it in a repo self-documents last updated date. Thoughts?

[EwoutH]

I think we can just archive them?

Archiving repositories
You can archive a repository to make it read-only for all users and indicate that it's no longer actively maintained. You can also unarchive repositories that have been archived.

[jackiekazil]

Agree