[DOCS] de translation

[bnbong]

Requesting Merging

Description

Part of #59
Refs #60
Closes #66

This PR completes the German documentation localization rollout for FastAPI-fastkit and aligns the related German translation policy/docs with the current localization workflow.

The main goal of this PR is to provide a complete German locale under docs/de/, improve readability and terminology consistency across translated pages, and clarify that the repository-root CHANGELOG.md remains the canonical English release history.

Type of Change

• BUG FIX
• ADDING NEW TEMPLATE
• FEATURE ADDED/UPDATED
• HOTFIX
• DELETING UNNECESSARY FEATURES
• DOCUMENTATION & DEVOPS
• Etc..

Test Environment

• Local macOS (Apple Silicon / M1), zsh
• Python via uv
• Verified command:
  • uv run mkdocs build
  • Result: Documentation built successfully

Major Changes

• Completed the German docs locale under docs/de/

  • Added or finalized German pages across:
    • top-level docs pages
    • user-guide pages
    • tutorial pages
    • contributing pages
    • reference pages
  • German locale now has 26 / 26 markdown pages present

• Improved German readability and terminology consistency

  • Reworked translation-style phrasing into more natural German
  • Standardized visible link labels, explanatory text, code-block comments, and policy wording across all phases
  • Kept commands, product names, template ids, file paths, and env vars in English where appropriate

• Updated translation policy and status docs

  • Refined the locale rollout guidance to match the current per-locale / phased workflow
  • Clarified English (docs/en/) as the source of truth
  • Updated German translation-status / translation-guide pages to reflect the current localization policy

• Clarified changelog handling for localized docs

  • Kept the repository-root CHANGELOG.md as the canonical English release history
  • Updated docs/de/changelog.md to act as a German entry point / wrapper for the canonical English changelog instead of maintaining a separate translated release history
  • Adjusted related wording in German policy/reference pages accordingly

• Refined German docs navigation and cross-page consistency

  • Preserved markdown structure, code fences, tables, admonitions, and internal links
  • Ensured translated pages remain aligned with the English source structure and current MkDocs i18n behavior

Screenshots (optional)

N/A

Etc

• This PR is documentation-focused and does not change runtime product behavior.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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