docs: rewrite README to follow the marketing landing page structure

- Lead with Execution, Filesystem, and Orchestration as top-level sections
- Restructure the quickstart into four numbered steps, with per-agent
  install commands and a pointer to agentos-core
- Replace the why/vs-sandbox framing with runtime mechanism and a
  "when to use a sandbox instead" section
- Collapse three benchmark tables into one, linking out for methodology
- Fold the feature list into a documentation index
- Fix dead links: /docs/benchmarks, /docs/js-runtime; drop /docs/webhooks

Author: NathanFlurry