Create proper documentation for MARR

[virtualian]

Summary

MARR currently lacks comprehensive user documentation. The README provides installation and basic usage, but there's no proper documentation covering:

• Getting started guide for new users
• Detailed explanation of the two-layer configuration system
• How to customize standards for specific projects
• Best practices and workflows
• Troubleshooting guide beyond npm permissions

Context

The docs/ directory currently contains:

• docs/product/prd.md - Product requirements (internal)
• Empty docs/implementation/ and docs/user/ directories

The README has grown to include troubleshooting but still lacks depth for users who want to understand the system fully.

Acceptance Criteria

• Create docs/user/getting-started.md - Step-by-step guide for new users
• Create docs/user/configuration.md - Detailed explanation of two-layer system
• Create docs/user/customization.md - How to customize/override standards
• Create docs/user/standards-reference.md - Documentation of each bundled standard
• Update README to link to detailed docs
• Ensure docs follow the documentation standard in prompts/prj-documentation-standard.md

Notes

Documentation should follow the role-first organization defined in prj-documentation-standard.md:

• docs/user/ for adopters using MARR
• docs/implementation/ for developers working on MARR itself
• docs/product/ for product strategy (already has PRD)