Docs: refresh README for 0.7.0

[ravenrs]

Brings the README in line with what shipped through 0.7.0 (docs-only — no release).

• Roadmap split into Delivered (projected elevations, interactive multi-rack editor, multi-rack designs) vs Planned (Apply, conflict detection, power projection, template-driven export).
• Intro reworded to match current behavior: live dcim.Device/dcim.Rack records stay untouched; each planned change is a structured placement (Apply still upcoming).
• Features dropped the stale "Stage 1 release" framing and now lists the delivered UI: interactive multi-rack editor, device-type catalog + per-user favorites, per-user rack visibility, projected/read-only elevations, and the rack-page panel.
• Screenshots placeholder reworded (points to the docs site) instead of "coming with the Stage 2 editor".
• Compatibility table version row updated 0.1.0 → 0.7.0 (range unchanged; full matrix stays in COMPATIBILITY.md).