feat(docs): add multi-language (i18n) support to the docs site — EN + TR #40

Summary

Add internationalization (i18n) to the documentation site (packages/docs) so
it can be read in multiple languages. The site has no i18n today — routing,
chrome strings, content, SEO, and search are all single-language (English).

• Phase 1 (required, in scope): English (en) and Turkish (tr),
  with en as the default.
• Phase 2 (future, out of scope here): Russian (ru), Chinese
  (zh, "CN"), and Korean (ko, "KR") — added once Phase 1 is in place;
  the system must be left open to them with no further architectural changes.

Current state (what i18n must touch)

• No existing i18n — <html lang="en"> is hard-coded (index.html).
• Routing: react-router-dom resolves pages from pageBySlug, built from an
  eager import.meta.glob('./content/**/*.mdx') keyed by frontmatter slug
  (src/pages.ts).
• Content: 73 MDX pages under src/content/ (getting-started,
  foundations/, guides/, components/, reference/, changelog/, index).
• Chrome strings (hard-coded in TSX): "On this page"
  (TableOfContents.tsx), "Page not found" (NotFound.tsx), the TopNav brand,
  the sidebar/primary nav labels and group names in src/data/navigation.ts,
  plus the search dialog and footer.
• SEO: useDocumentHead + src/seo/vite-plugin-seo.ts prerender per-route
  head tags — English-only, no hreflang.
• Search: src/search/vite-plugin-search.ts builds one MiniSearch index over
  English content text.

How to implement

Build a lightweight, bespoke i18n (no heavy dependency) consistent with the
existing hand-rolled Vite/MDX setup.

1. Locale config — single source of truth

• Add src/i18n/config.ts exporting locales = ['en', 'tr'] as const,
  defaultLocale = 'en', and a Locale type. Everything (routing, switcher,
  SEO, search, content glob) reads this list, so Phase 2 is "append to the
  array + add files", nothing else.

2. Routing — default locale unprefixed, others prefixed

• en stays unprefixed at /...; other locales are prefixed: /tr/...
  (e.g. /tr/components/button). SEO- and migration-friendly (existing English
  URLs don't change).
• Add a LocaleProvider that derives the active locale from the pathname;
  update slugFromPath (src/pages.ts) to strip a leading locale segment before
  lookup. Unknown/!translated slug → fall back to the en page (see §5).

3. Locale detection, persistence, <html lang>

• On first visit, pick from navigator.language (matched against locales),
  else defaultLocale; persist the user's explicit choice in localStorage.
• An effect sets document.documentElement.lang to the active locale on every
  navigation/locale change (replacing the hard-coded index.html value).

4. Chrome string catalog

• src/i18n/messages/en.ts + tr.ts, each implementing a shared typed
  Messages interface (compile error if a key is missing in any locale).
• An I18nProvider exposes a useT() hook returning the active catalog.
• Replace every hard-coded UI string in TopNav, Sidebar, Footer,
  TableOfContents, SearchDialog, NotFound with t(...). Nav group
  labels currently come from GROUP_ORDER/frontmatter, so introduce stable
  group keys and translate them through the catalog (not the raw label).

5. Content — per-locale trees with EN fallback

• Reorganize src/content/ into per-locale trees: content/en/**,
  content/tr/**. Update the glob in pages.ts to capture the locale from the
  path and build the locale-prefixed slug.
• pageBySlug resolves the active locale first, then falls back to the en
  page when a translation is missing, rendering a subtle "not yet translated"
  banner (a small MDX/shell component). This lets TR ship incrementally.
• Translate the core pages to TR first (landing, Getting Started, Foundations,
  Guides); component reference pages may fall back to EN initially and be
  translated over time.

6. Language switcher (UI)

• A switcher in TopNav built with a Manti component (Menu or Select,
  not raw HTML) that navigates to the same slug in the target locale and
  persists the choice.

7. SEO

• useDocumentHead + vite-plugin-seo: per-locale <title>/description,
  hreflang alternate links between locale variants of each page, correct
  <html lang>, and per-locale entries in the sitemap. The prerender step
  iterates locales × pages.

8. Search

• Make vite-plugin-search emit one MiniSearch index per locale;
  SearchProvider selects the index for the active locale so results match the
  language being read.

Phase 2 (future — not in this issue)

• Add ru, zh, ko by appending to locales in src/i18n/config.ts and
  dropping in their message catalogs + translated content trees. If Phase 1 is
  built config-driven, no further architectural changes are required.

Acceptance criteria (Phase 1)

• en and tr selectable via a Manti-component language switcher in TopNav.
• Initial locale auto-detected, choice persisted, <html lang> updated live.
• Locale-aware routing (/tr/...) with EN fallback + "not yet translated"
  notice for untranslated pages.
• All chrome strings come from typed per-locale catalogs (no hard-coded UI
  copy left in shell components); missing keys fail typecheck.
• TR translations for the core pages; component pages fall back gracefully.
• SEO emits hreflang + per-locale titles/sitemap; search is per-locale.
• Adding a locale is a single-array change (proves Phase 2 readiness).
• pnpm --filter @manti-ui/docs build passes.