docs: add README quickstart

[vku2018]

Closes #82

Summary:

• Add a newcomer quickstart covering NuGet install, local demo run, Docker demo run, and minimal configuration guidance.
• Link the deeper demo setup and multi-backend configuration docs from the quickstart.
• Update stale README links that still pointed to the old repository location.

Validation:

• git diff --check
• npx --yes markdownlint-cli README.md
• Not run: dotnet build (local SDK is 8.0.422; repository projects target net10.0)