added overview and diagram

[fuddin-bit]

What does this PR do?

Fills in the missing README sections such as Overview and Architecture sections that were previously TODOs:

• Describes what llm-d-latency-predictor does and how the training/prediction servers fit into llm-d predicted latency-based scheduling
• Adds an architecture diagram from the https://drive.google.com/drive/folders/1-CyochqqoMccMrZ7BFYPZwg1XRHdS4DY?dmr=1&ec=wgc-drive-globalnav-goto

Why is this change needed?

The README left Overview and Architecture as placeholders for contributors, as they need a clear description on how the latency-predictor fits into the overall well-lit baseline path.

How was this tested?

• Unit tests added/updated
• Integration/e2e tests added/updated
• Manual testing performed

Docs-only change; verified the README renders the new overview text and architecture image correctly.

Checklist

• Commits are signed off (git commit -s) per DCO
• Code follows project contributing guidelines
• Tests pass locally (make test)
• Linters pass (make lint)
• Documentation updated (if applicable)

Issues

#55