docs(readme): persona-first restructure of the README top (#156)

[takashikasuya]

Addresses #156.

⚠️ Stacks on #170 (#160). This PR's new top block links to docs/concepts.md, which is added
in #170. Merge #170 first, then this one — otherwise the two docs/concepts.md links dangle
until it lands. (Once #170 merges to main, this PR's diff reduces to just the README change.)

Summary

Reworks the first screenful of the 690-line README so a first-time reader immediately sees
what it is → what it looks like → the first command → where to go for their goal, instead of
hitting Breaking-change / Azure-compat / IaC detail up front (the #156 acceptance criterion:
"README冒頭スクロール1画面以内に「何か・見た目・最初の1コマンド」が収まる").

New top section (right after the intro):

• 📸 Screenshot/GIF placeholder — an HTML-comment slot + a text pointer to the 3 core screens
  (/resources tree → /points/{id} chart → control dialog). Actual captures need a live UI, which
  this automated environment can't run, so they're explicitly left for a maintainer (marked in-file).
• 🚀 5-minute quickstart — 2 commands + login.
• ✨ 主要機能 — feature bullets.
• 🧭 目的別の入口 — the persona table the issue asked for (A demo / B MQTT / C gateway-dev /
  D twin / E prod / F API), each linking straight to the relevant doc.

Moved "migration / compat / IaC" detail out of the top (the other acceptance criterion):

• The Timescale→Parquet Breaking-change block and the Azure IoT Hub block in the architecture
  header are collapsed to one-line pointers. Their full content already lives in the linked docs
  (oss-warm-parquet-lake.md / oss-tier-architecture.md; system-architecture.md already documents
  the Kandt/IoT Hub bridge), so nothing is lost.
• Removed the duplicated legacy host-run steps (§5/§6/§7 — they repeated §4); folded the one unique
  detail (WithLocalAuth) into a pointer to getting-started.md. Retitled the detailed quickstart to
  「起動手順（詳細）」.
• Mirrored the persona table + screenshot note in the English summary.

Test plan

• All docs/ links in the README verified to resolve (the only "missing" hit is the example path
  inside the screenshot HTML comment — inert, not rendered).
• Markdown code-fences balanced (40, even); the MQTT intra-page anchor target header exists.
• No information loss: every demoted/removed block's content is preserved in a linked doc or a
  concise pointer.

Checklist

• I read CONTRIBUTING.md and followed the coding conventions in CLAUDE.md
• I added or updated tests — n/a (docs-only)
• I updated relevant docs (this PR is the README)

Out of scope (possible follow-ups): real screenshots/GIF capture; relocating the lower-half env-var /
ports / deploy reference tables into dedicated docs (they already carry doc pointers and sit below the
fold).

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