docs: add root README with repository layout and setup guide

[profitegood]

What type of PR is this?

docs

Summary

Adds a root-level README.md that GitHub renders on the repository home page.
Documents the purpose of every top-level file and directory, prerequisites,
and step-by-step setup and run instructions. Closes #<номер issue>.

How to test

1. Open README.md on the PR's "Files changed" tab and verify Markdown renders correctly.
2. Follow the "Setup — Step by Step" section on a clean checkout to confirm the flow works end-to-end.
3. Verify all internal links (./docs/architecture.md, etc.) resolve.

[profitegood]

Hi @ewheeler @gachagas - this is a docs-only PR adding a root README.md (currently only docs/README.md exists, which GitHub doesn't render on the home page). Could you take a look when you have a minute? Thanks!