docs: rewrite homepage to lead with positioning + add /about and /why-etherpad

[JohnMcLear]

Summary

Replaces the homepage's generic feature-description copy with positioning copy organised around three differentiators — authorship visibility, sovereign hosting, and structural malleability (including AI as opt-in plugin rather than forced default). Adds two new content pages so the site has somewhere to send visitors who want to read more.

Companion to ether/etherpad-lite#7526, which makes the same positioning shift on the README.

Why

The current site reads as a feature description that any of a dozen real-time editors could share. The positioning shift answers what Etherpad is for before what it does, which is what institutional evaluators (sovereignty-conscious orgs, schools, public sector, civic-tech, journalism) actually need before they self-host. None of this changes the product — only how it's described.

Changes

Homepage (app/page.tsx)

• MainHeadline — new hero: "Etherpad — the editor for documents that matter" with a value-prop subhead and three CTAs (manifesto, why-etherpad, self-host). Stats bar gets a section title and per-stat captions so the numbers feel anchored.
• RealTimeCollaboration — rewritten as "Three things that make Etherpad different" with three short sections (authorship / sovereignty / malleability) and Font Awesome icons.
• WhoUsesEtherpad (new component) — categorical adopter list (Wikimedia, EU public sector, schools, civic-tech, newsrooms, self-hosters) so institutional evaluators have proof points without leaving the page.
• AIOnYourTerms (new component) — explicit positioning on AI as opt-in plugin rather than forced default, aimed at regulated industries.
• Contribute — restructured into four clear paths (contribute code/docs, become a maintainer, use + evangelise, how a contribution lands).
• Footer — deemphasises the BTC address (moved lower, smaller, lower opacity). Pure perception fix for institutional visitors. Bitcoin remains accepted, just doesn't read as a relic.

New routes

• /about — full manifesto: "An editor for documents that matter." Sections: the problem, the documents that matter (constitution / treaty / scientific paper / journalism / school examples), what Etherpad is, our principles, what we won't do, sixteen years of holding the line, what you can do.
• /why-etherpad — honest comparison page. Capability matrix vs. Google Docs, MS 365, Notion, CryptPad, HedgeDoc. Followed by "when you should use something else" (CryptPad for E2E, HedgeDoc for markdown, Onlyoffice/Collabora for office suite, Google/Notion for polish, Git for cryptographic provenance) and "when you should use Etherpad".

What this PR does not change

• No dependency changes.
• No build configuration changes.
• No behaviour changes — static export still works (pnpm run build verified locally; all routes generate cleanly).
• No visual redesign — existing layout, styling, and components reused.
• Header navigation unchanged for now (could add /about and /why-etherpad to the nav in a follow-up; for v1 they are reachable via hero CTAs).

Test plan

• Render the homepage locally and confirm hero, three pillars, who-uses, AI-on-your-terms, contribute restructure all render correctly in light and dark mode.
• Visit /about and /why-etherpad directly and confirm both render with header + footer.
• Confirm the comparison table on /why-etherpad is readable on mobile (it has horizontal overflow scrolling enabled but worth checking on a real device).
• Confirm the categorical "Who uses Etherpad" list reads accurately to maintainers familiar with the actual deployment landscape; soften phrasing if needed before merge.
• Verify the BTC address is still present in the footer (deemphasised, not removed).

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