docs: Add high-level documentation on concept for signstar

Add documentation explaining the previous setup, which is to be supplanted by signstar. Explain core concepts of signstar.

Adapt top-level README to new in-depth documentation and details.

Easiest to preview with rendered mermaids in https://gitlab.archlinux.org/archlinux/signstar/-/tree/docs/concept/resources/docs and https://gitlab.archlinux.org/archlinux/signstar/-/tree/docs/concept/README.md