Rewrite homepage "Achievements & Awards" as a genuine Impact section

## Problem

The homepage's `content/home/5-achievements.md` section (just rebuilt visually in #814, replacing a yearly-regenerated infographic image with a CSS-based list) still carries its old content, which is a mix of:

- **Vanity/activity metrics** — social followers, Slack members, subscriber counts, contributor counts.
- **A bulleted list of awards FORRT has received** (Dorothy Bishop Prize, OSC Award, Open Scholarship Prize), which duplicates `/awards`.

None of this actually answers "what has FORRT changed?" — e.g. how many educators have adopted FORRT materials, what policy/institutional change the policy briefings drove, what's shifted in teaching practice as a result of the Educational NEXUS, etc. That's a genuine **impact** narrative, and it's currently missing.

## Naming collision to resolve deliberately

"Impact" is already used for two other, unrelated things on the site:

- `/awards` — page titled **"Awards & Impact"**.
- `/impact` (nav: Educational NEXUS → **"Impact of OS on students"**) — a specific published review (Pownall et al., 2023, *Royal Society Open Science*) on the evidence for teaching open/reproducible scholarship, unrelated to FORRT's own organizational impact.

If the homepage section becomes "Impact", we'd have three differently-scoped "Impact" surfaces. Whoever picks this up should decide the IA explicitly (e.g. rename/merge one of the existing pages, or pick a distinct label for the homepage section) rather than adding a fourth ad-hoc meaning.

## Proposal

1. Decide what "impact" means for the homepage section and how it relates to `/awards` and `/impact` (rename, merge, or clearly differentiate labels/scope).
2. Rewrite the copy to lead with concrete outcomes (reach, adoption, institutional/policy influence, evidence of behavior change) rather than follower/member counts.
3. Move the awards bullet list to live solely on `/awards` (linked from the homepage, not duplicated).
4. Decide whether any of the current milestone stats (Slack members, newsletter subscribers, contributors, projects, partnerships) still belong in this section as supporting evidence, or should move/shrink.

This is primarily a **content/messaging** decision — implementation (the CSS-based milestones list from #814) can be reused once the copy and structure are agreed.

_Raised while rebuilding the homepage Achievements & Awards section (#814)._

content/home/5-achievements.md から始め、既存の /awards ページと /impact ページを比較して、それぞれの範囲と命名を把握します。次に、ホームページのセクションの情報アーキテクチャを決め、具体的な組織上の成果を中心に書き直し、重複している受賞内容を削除し、補足的な根拠として残すマイルストーン統計を決定します。

索引モデルが issue の本文から書いたものです。