docs: make Docker the default pipeline path in the README quickstart

[flamboh]

Problem

The README quickstart still walked new users through the native toolchain — submodule init and a local nfdump build — even though the Docker path landed in #98. That is the most error-prone part of setup, and it was the first thing a newcomer hit.

Solution

The quickstart now uses scripts/netflow-db-docker.sh by default:

• Step 1 drops git submodule update --init --recursive and ./vendor/scripts/compile-nfdump.sh; it is just clone plus bun install.
• Step 3 runs the Docker wrapper with --capture-root, and points out that the path is the same absolute root_path from step 2 — the mount and the dataset config have to agree, which is the one thing that will bite people.
• A one-line pointer to native setup for anyone who would rather build the toolchain themselves.
• The intro tool list now reads "Bun and Node.js run the dashboard, and Docker runs the pipeline" instead of naming the nfdump build tools.

Flows to exercise

Read the quickstart top to bottom as a new user with Docker installed and no Rust toolchain: steps 1 through 4 should complete without touching the native sections. The --capture-root value and root_path in datasets.json must be the same absolute path.

Verification

bun run format passes. The #native-setup anchor resolves against the current docs/user/setup-pipeline.md. No behavior change — documentation only.

The docker build itself remains unverified on my host (no docker-group access), as noted in #98.

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Authored with Claude Code (Fable 5).