Create comprehensive MARR documentation (#29)

[virtualian]

Summary

• Add user documentation: getting-started, configuration, customization, standards-reference
• Add developer documentation: contributing, testing, publishing, specification, architecture
• Restructure docs with role-first + Diátaxis organization (user/dev → how-to/reference/explanation)
• Set up Docsify for GitHub Pages publishing
• Consolidate and delete outdated dev docs
• Update README with documentation links

Documentation Structure

docs/
├── user/
│   ├── how-to/
│   │   ├── getting-started.md
│   │   └── customization.md
│   ├── reference/
│   │   └── standards-reference.md
│   └── explanation/
│       └── configuration.md
└── dev/
    ├── how-to/
    │   ├── contributing.md
    │   ├── testing.md
    │   └── publishing.md
    └── explanation/
        ├── specification.md
        └── architecture.md

Test plan

• Run docsify serve docs locally and verify navigation works
• Verify all internal links resolve correctly
• After merge, enable GitHub Pages from /docs folder

Closes #29