Update Vision section with latest changes.
- Make the vision diagrams consistent with the terminology section: - Output is on the right, input is on the left. - Use colors consistently. - Rename "resource" to "artifact locator". - Simplify the diagram to reduce confusion (fixes #31). - Update the level explanations based on recent changes: - SLSA 1 is unsigned. - Add SLSA 1.5 (merged with the SLSA 2 section). - Minor wording updates. - Remove Deployment Policies section. We will eventually need to explain policies, but for now let's omit it until we agree on what that should look like.