docs(deepagents): simplify README (#456)

## Changes

### `libs/deepagents`

- Rewrote `README.md` from an exhaustive reference style to a minimal
structure:
  - concise overview and "What's included"
  - short install + quickstart example
  - brief customization example
- LangGraph-native note, value proposition bullets, docs/resources, and
security section
- Updated top docs anchor link to JavaScript Deep Agents docs.
- Changed `<picture>` fallback `<img>` source from `logo-dark.svg` to
`logo-light.svg` for better npm/default rendering.
- Removed duplicated deep-dive sections and verbose option-by-option
content in favor of docs links.