docs(readme): simplify user guide

[skulidropek]

Summary

• rewrite the root README into a short Russian user guide
• keep only the product essence, prerequisites, installation, and core commands
• remove long technical sections from the root README and leave details to docker-git --help

Verification

• checked published install path for @prover-coder-ai/docker-git
• verified example commands against current docker-git --help

Documentation-only change.