docs: add QUICKSTART guide, update ARCHITECTURE and API docs

[chaosreload]

Summary

Add documentation for the virtio-blk filesystem emulation feature (companion to the upcoming feature PR).

Changes

• docs/QUICKSTART.md (new): End-to-end setup guide covering machine provisioning, Docker-based rootfs build, Firecracker install, snapshot creation, and API testing. Validated on a fresh EC2 c8i.xlarge instance.
• docs/ARCHITECTURE.md: Add "Virtio-Blk Filesystem Emulation" section explaining the MMIO emulator, OverlayBlockDevice CoW design, and guest init protocol.
• docs/API.md: Document the execution protocol and filesystem access via virtio-blk.
• README.md: Add benchmark results and filesystem access section.

Notes

This is a documentation-only PR. No code changes. The feature implementation will follow in a separate PR.