docs: source-align stable and next Echo website

[vishr]

Summary

Build stable (/) and next (/next/) documentation from separately pinned Echo revisions, with separate search indexes and visible source/version switches. Keep the website and documentation tooling in echox; Echo itself remains the API source.

• Extract config fields, types, deprecations, signatures, and line links from the pinned Echo revisions. Cover all 22 core middleware config types across 20 pages and all five locales.
• Extract JWT, Prometheus, and OpenTelemetry reference facts from their own pinned module releases. Show the owning module and version on each page.
• Compile the exact Request Logger and Static example files used by the docs against the pinned Echo source. Keep behavior, defaults, security advice, and task guidance as authored prose.
• Add a middleware task page in every locale, plus route/anchor, locale, image text, search asset, and static size checks. Keep existing cookbook and guide routes and redirects.
• Track English and localized edits by section for the new task page and the two reviewed example pages. A stale section now fails the site build; matching hashes do not certify prose quality.

Verification

• go test ./... in the repository root: passed.
• npm run build: passed for stable and next, including source/external API baselines, example compilation, translation freshness, 676 routes, 59,608 internal links/assets, and representative HTML/asset size budgets.
• Offline lychee audit with fragments over the combined site: 67,998 links, 0 errors.
• git diff --check: passed.

Review focus

Please review the authored task guidance and localized wording, and check behavior explanations against both pinned Echo revisions. The extractor establishes API facts, not runtime defaults or security behavior. Static size checks are regression budgets; browser Core Web Vitals and a native-speaker review are separate quality checks before publishing.

Tracks #448. Echo cleanup PR labstack/echo#3119 has already merged. The Echo-side cross-repository gate remains a separate maintainer decision.