[Docs][Mojo] Move some of the Mojo notebooks into new "public" dir. (…

…#12881)

This is to hopefully avoid committing notebooks that are meant for
internal eyes only. The Mojo Playground can simply fetch all notebooks
from this dir, and so can the docs website to render them as static web
pages (for people without Playground access to read).

modular-orig-commit: e00f2cb1c44c408eeb54918c1a722df0f9b7fa7d