Restructure documentation with Diátaxis framework (issue #40)

[virtualian]

Summary

• Reorganize docs by user role (users/, contributors/) then content type (how-to/, reference/, explanation/) following Diátaxis principles
• Old docs preserved in deprecated/docs-v1/ for reference
• Update MARR standards to require Diátaxis content types and explicit "no changes on main" rule

New Structure

docs/
├── README.md
├── users/
│   ├── how-to/
│   │   ├── getting-started.md
│   │   ├── scan-projects.md
│   │   ├── validate-packages.md
│   │   └── integrate-with-ci.md
│   ├── reference/
│   │   └── command-reference.md
│   └── explanation/
│       └── threat-model.md
└── contributors/
    ├── how-to/
    │   └── contributing.md
    ├── reference/
    │   └── architecture.md
    └── explanation/
        └── design-decisions.md

Closes #40