[DOCS, I18N] German documentation localization

[bnbong]

Requesting other issue

Track documentation localization for German (Germany).

Why

This locale is already configured as a built docs target, but translated coverage is still partial. This issue tracks all localization work for this language in one place, while allowing the work to land in multiple small PRs.

Target directory

• docs/de/

Translation rules

• English (docs/en/) is the source of truth
• Preserve markdown structure, headings, links, code fences, tables, and admonitions
• Keep commands, file paths, env vars, template ids, and product names in English
• Do not silently change technical meaning from the English source
• If a sentence in the English docs is unclear, ask in the issue or PR instead of guessing
• Update translation status only when translated pages actually land

Suggested rollout inside this issue

Phase 1: Core user-guide pages

• docs/de/index.md
• docs/de/user-guide/installation.md
• docs/de/user-guide/quick-start.md
• docs/de/user-guide/creating-projects.md
• docs/de/user-guide/using-templates.md
• docs/de/user-guide/choosing-a-starter.md
• docs/de/reference/translation-status.md

Phase 2: Remaining user-guide and tutorial pages

• docs/de/user-guide/adding-routes.md
• docs/de/user-guide/cli-reference.md
• tutorial pages under docs/de/tutorial/

Phase 3: Contributing / reference / changelog pages

• contributing pages under docs/de/contributing/
• reference pages under docs/de/reference/
• docs/de/changelog.md

Non-Goals

• Translating everything in one PR (it's ok, but not recommended)
• Rewriting English source docs as part of translation work
• Product/runtime changes

Acceptance Criteria

• Translation work for this locale is tracked in this issue
• Pages are translated in small reviewable PRs
• uv run mkdocs build passes for translation PRs
• Translated links resolve correctly
• Translation status reflects the actual landed coverage