docs: add beginner-friendly documentation with visual guides (#66)

New documentation for developers unfamiliar with the project:

GETTING_STARTED.md (374 lines)
- 5-minute quick start guide
- Simple ASCII architecture diagram
- Connection examples for Python, Node.js, Go, Rust
- Daily workflow commands with expected output

GLOSSARY.md (346 lines)
- Plain-language definitions for all key terms
- Organized by category (Core, Vault, Database, Redis, etc.)
- Simple diagrams for complex concepts like PKI hierarchy

LEARNING_PATHS.md (470 lines)
- 8 structured learning paths by goal/role
- Step-by-step reading order with time estimates
- Paths: Quick Start, Understanding, Developer, Vault, Observability,
  Troubleshooting, DevOps, Contributing

CLI_REFERENCE.md (723 lines)
- Consolidated command reference from scattered docs
- Quick reference card at top
- All commands with options, examples, and expected output
- Exit codes and environment variables

USE_CASES.md (723 lines)
- 7 task-driven walkthroughs with simple diagrams
- Connect apps (Python/Go/Node.js) to databases
- Use Vault secrets, set up monitoring, run tests

TROUBLESHOOTING.md updates (125 lines added)
- 5 focused Mermaid flowcharts (bite-sized, not comprehensive)
- Decision trees for: startup issues, database connections,
  Vault sealed, Redis cluster, container restarts

README.md updates
- Added "New to DevStack? Start Here" section
- Links to all beginner documentation

Design principles applied:
- Small, focused diagrams over comprehensive ones
- Visual learning emphasized throughout
- Plain language, no jargon without explanation
- Task-driven structure for practical use

Author: NormB