docs: label the project as a demo and split the README

The README had grown to 934 lines and nowhere said that this is a
demonstration rather than something to deploy. The caveats existed but sat
at line 886, past the point anyone reads.

Adds a banner directly under the title, and moves the three longest
sections into docs/ with summaries left behind, so the README covers what
the project is and how to run it while the detail stays one click away.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AKgfqZwc1ZragsJwwkFcdz

Author: mecemis