docs(mintlify): make the site English-only, document the auto-localization path

[CodeWithJuber]

What & why

The Mintlify docs site carried five hand-maintained translated locale trees
(ar, hi, cn, zh-CN, zh-Hans). Keeping parallel prose in sync by hand was the
main source of documentation drift — every English fix had to be re-applied five times,
and any miss left the site lagging the code.

This makes the site English-only:

• Removes the five translated locale trees (95 .mdx files) and reduces
  mintlify/docs.json navigation.languages to a single en entry.
• Documents in mintlify/README.md how to bring translations back the automated way
  instead of hand-maintaining parallel trees: Mintlify's built-in AI auto-localization
  (write English once, list locale codes in docs.json, enable translation in the
  dashboard) plus an optional code→docs Workflow.
• The English-scoped checkMintlify drift guard in src/docs_check.js is unchanged and
  keeps working regardless of how many display languages are enabled.

No source/runtime code changed — docs and config only.

Checklist

• npm test passes (Node 18/20/22) — 1072 pass / 2 skip
• npm run check passes (Biome lint + format) — markdown is not linted
• New public functions have a test — n/a, no code change
• Conventional commit message (feat:/fix:/docs: …)
• CHANGELOG.md updated under ## [Unreleased]
• No new runtime dependency (dev deps ok)
• Substrate/docs updated if this changes forge substrate, forge impact, router/gate, or MCP substrate tools — n/a

Risk & rollback

• Risk level: low
• Rollback plan: revert this commit — the deleted locale trees are restored from git history; no code paths depend on them.

Extra checks (tick if applicable)

• npm run typecheck passes
• Input validated at boundaries; errors handled (no swallowing) — n/a, no code change
• Authorization/ownership checked (if it touches access) — n/a
• Logs contain no secrets/PII
• node src/cli.js docs check green (English-scoped Mintlify guard) + mintlify broken-links clean

🤖 Generated with Claude Code

https://claude.ai/code/session_01C7htTiesXKLvAUtv6ET2jz

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Generated by Claude Code