#3280 docs: publish translated docs in twelve languages and the tool that maintains them

Ships the docs in twelve languages — de, es, fr, hi, ja, ko, pt (Brazilian), ru,
tr, uk, zh and zh-hant, each at /<code>/ — machine-translated from the English
pages, plus the small tool that keeps them current. English stays
the only source; everything under i18n/<lang>/pages/ is generated. Supersedes
#3231 and #3265 (same goal; this takes the review of both into account —
notably no anchor edits to the English docs, no nav-label subsystem, no
second-model gate, no state file).

• scripts/docs/translations.py — three commands: status, translate --lang X [--pages …], stage [--lang X]. Pages are split at ## sections; only sections whose English changed are re-translated
  and everything else is carried forward byte-for-byte, so a one-line English
  edit is a one-line translated diff. Heading ids come from the site renderer
  itself and are pinned into the translation; code blocks are re-imposed from the
  English mechanically and link targets are checked against it. Provenance (section hashes) lives
  in each generated page's front matter — no side state.
• Corrections are inputs: i18n/<lang>/instructions.md (register, voice,
  typography) and glossary.json (terms, keep-list, banned renderings), then
  translate --pages … for the affected pages; the generated pages are never
  hand-edited.
• Build: language sites are staged (English overlaid with translations, a
  short notice after each H1 — machine-translated / behind the English page /
  shown in English), sidebar titles come from each translated page's own H1,
  and language builds are non-strict (missing snippet files included), so an
  English-only PR can't fail because of translations; a page whose English has
  changed since is served exactly as generated under an "outdated" notice until
  refreshed, and a page never translated is shown in English. The language sites
  add about a minute to the docs build in total; the API reference is linked,
  not rebuilt per language.
• The language switcher shows code - name and keeps you on the current page
  when you change language. docs/translations.md explains all this to readers.

Intentional behavior changes

None to the SDK. The docs build now also produces the twelve language sites;
anthropic is added in a non-default translate dependency group (normal
installs don't get it).

Review notes

• Registers/terminology are provisional pending native review — Portuguese has a
  reviewer lined up; reading order: i18n/general-prompt.md →
  i18n/pt/instructions.md + glossary.json → a few pages under
  i18n/pt/pages/ → docs/translations.md → the tool.
• Known follow-ups: CJK search segmentation needs a Zensical bump (blocked on
  packaging, tracked separately); French spacing uses ordinary rather than
  no-break spaces (the model won't emit U+00A0; a small deterministic pass could); the "API Reference" nav label stays English
  on language sites; a handful of terminology consistency notes for native
  reviewers are listed in i18n/<lang>/glossary.json notes as provisional.
• Running the tool needs ANTHROPIC_API_KEY; nothing in CI calls the API.
• What the tool checks mechanically is deliberately bounded: heading skeleton
  and ids, code blocks, link targets, inline code, admonition/tab markers and
  list/table counts, per section. Placement within a section, glossary
  keep-terms, block titles and anything else about the prose is left to review
  of the (ordinary, human-reviewed) PR that commits regenerated pages.

_{AI Disclaimer}