Upgrade to Docusaurus 3, standardize on Yarn, improve SEO, and invalidate stale 2.x translations

[imorland]

A combined modernization pass on the docs site. All yarn build output verified across the default version and all 7 locales.

Docusaurus 3 upgrade

• @docusaurus/* 2.3.1 → 3.10.1, React 17 → 18, MDX 1 → 3, prism-react-renderer 1 → 2 (named-export theme import), clsx/@svgr/webpack bumps.
• Drops the webpack overrides pin added in Fix broken production docs deploy (webpack ProgressPlugin ValidationError) #529 — no longer needed on v3. The webpackbar/ProgressPlugin schema mismatch that forced the pin doesn't exist on Docusaurus 3.
• Migrate onBrokenMarkdownLinks to the v3 markdown.hooks location.
• Escape MDX-v3-breaking literals ({user}, <=0.1.0, a stray <) in English, versioned, and locale content so every build compiles.

Tooling

• Standardize on Yarn (matches the rest of the Flarum ecosystem): regenerate yarn.lock, remove package-lock.json. CI uses yarn install --frozen-lockfile + yarn build with yarn caching.
• Workflow Node 20.x → 24.x (the deprecated-Node-20 warning); actions/checkout@master → @v4.
• README: Node ≥20, Yarn commands, and a new Translations section documenting the Crowdin workflow.

SEO

• Fix url to https://docs.flarum.org (was flarum.org) — this one change corrects canonical, og:url, the sitemap domain, and hreflang across the whole site.
• Default social card via themeConfig.image (Flarum banner) → og:image / twitter:image (summary_large_image).
• Site-level WebSite/Organization JSON-LD via headTags.
• Per-page TechArticle + BreadcrumbList JSON-LD for rich search results, via a small local postBuild plugin (plugins/structured-data) — no theme swizzle, works across all locales, upgrade-safe. Injects into all 148 pages per build.
• onBrokenAnchors: 'warn' (surfaces ~9 pre-existing broken in-page anchors).

Translations — 2.x invalidated

The 2.x (current) translations for all 6 locales were frozen at a Dec-2024 snapshot, ~18 months and 40+ commits behind English, and translated content that has since been corrected (e.g. the pre-Symfony Swiftmailer mail-driver API). With Crowdin dormant (~19 months of unmerged sync PRs), there was no path to fix them, and serving confidently-wrong localized 2.x docs at the 2.0 launch is worse than falling back to English.

• Removed i18n/<locale>/docusaurus-plugin-content-docs/current/ for all locales (~450 files). Docusaurus automatically falls back to English for untranslated 2.x pages — no 404s.
• Kept 1.x doc translations, code.json, and theme/UI translations.
• Commented out the 2.x source mapping in crowdin.yml so it isn't regenerated; re-enable to resume 2.x translation from current English.

Notes

• The default docs version (currently 1.x) is intentionally not switched to 2.x here — that's deferred to the 2.0 stable release.
• JSON-LD is now present where Docusaurus emits none by default; verified parseable.