Add contributing guidelines and enhance README

[soleil-colza]

Note:

• We still need to add some visual resources
• Pick a template section needs to be modified once we update the template schema

[greptile-apps]

Greptile Summary

This PR adds a CONTRIBUTING.md with repo layout, dev setup, and PR guidelines, and substantially rewrites README.md with a logo header, badges, expanded feature sections, and an architecture overview. Two SVG logo files (logo.svg, logo-dark.svg) are added and correctly wired up via a <picture> element for automatic light/dark switching.

Confidence Score: 5/5

Safe to merge — documentation-only changes with one minor cosmetic nit.

All changes are documentation and static assets. The only finding is a trailing separator character in the README nav bar, which is P2. No logic, security, or correctness issues are present.

No files require special attention.

Important Files Changed

Filename	Overview
README.md	Major rewrite: adds logo/badges, expands quickstart, adds Why Arkor, What Works, API taste, Architecture, and Repository sections; minor trailing separator nit in nav bar.
CONTRIBUTING.md	New file: well-structured contributing guide with repo layout, dev setup, PR guidelines, and security reporting; Node.js version inconsistency vs README badge already flagged in a prior review thread.
assets/logo.svg	New light-mode logo SVG; paths use default black fill, appropriate for light backgrounds.
assets/logo-dark.svg	New dark-mode logo SVG with fill="#ffffff"; properly paired with logo.svg via a element in README.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Developer: pnpm create arkor my-arkor-app] --> B[Scaffolded project]
    B --> C[pnpm dev → arkor dev]
    C --> D[Hono server on 127.0.0.1:4000]
    D --> E[Vite + React Studio SPA]
    D --> F[Hot-reload TypeScript src/arkor/]
    F --> G[createArkor registers trainer/deploy/eval]
    G --> H[arkor build → .arkor/build/index.mjs]
    H --> I[arkor start → runs on managed GPUs]
    I --> J[SSE: checkpoints stream back]
    J --> K[callbacks.onCheckpoint / onCompleted fire in-process]

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[k-taro56]