docs: add E2B API compatibility architecture proposal

This commit adds architecture design proposal for E2B API compatibility:

- docs/design/e2b-api-architecture.md - E2B API Phase 1 architecture design

This document describes the architecture and design decisions for
implementing E2B-compatible REST API and Templates management in AgentCube.

Signed-off-by: MahaoAlex <[email protected]>

Author: MahaoAlex