docs: split DAG Pipes manual into 3 parts

Part 1: CLI Guide — stacker pipe commands, no curl needed
Part 2: Visual Editor — drag-and-drop web UI walkthrough
Part 3: REST API Deep Dive — curl scripts, gRPC, full API reference

Original file becomes a short index linking to all three parts.
Same 3 examples across all guides (contact form → telegram/slack/email).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: vsilent