Create a Sigstore Landscape

Sigstore is a project that is part of OpenSSF. This landscape would represent Sigstore's ecosystem (and eventually be embedded in the OpenSSF main landscape). Goals for the landscape:
- Highlight the sigstore ecosystem of projects and services (make discovery easy)
- Have a central place where folks can see projects adopting sigstore
- Make it easy for folks to contribute their own case studies/adoptions
- Highlight integrations, end user case studies, etc.
- Make it easy to find guides to the various parts of the ecosystem

Ideally l.sigstore.org would be hosted at https://github.com/sigstore/sigstore-landscape
It should include a guide in the first version
The initial layout is loosely based on l.graphql.org
Here's a sketch: