docs: redesign README with architecture and exporter rationale

[rokernel]

Summary

• Redesign the root README to be cleaner and more structured for first-time users.
• Explain why exporters are useful in SONiC observability workflows and what this project solves.
• Add architecture diagrams, a clearer collector matrix, and explicit upstream acknowledgments.
• Add docs/architecture.md with a focused structural and runtime overview.

Validation

• go test ./... passed.
• go build ./... passed.
• markdownlint is not installed in this environment, so markdown linting could not be executed.