feat: add multilingual support with German and English

Implement complete multilingual support using Hugo's multilingual mode with German (de) as default and English (en) as secondary language.

Changes:
- Configure Hugo multilingual mode with language-specific parameters
- Reorganize content using language suffix pattern (filename.de.md, filename.en.md)
- Translate all content to English (home page and CV page)
- Implement language switcher that maintains page context
- Add language-specific navigation menus
- Update templates to use language-aware URL functions (relLangURL)
- Fix navbar brand and Home links to respect current language
- Add translationKey to content frontmatter for explicit translation linking

URL Structure:
- German: /de/ (home), /de/cv/ (CV page)
- English: /en/ (home), /en/cv/ (CV page)

Key Implementation Details:
- Use filename.{lang}.md pattern instead of {lang}/filename.md subdirectories
- Always use relLangURL for internal navigation, never .Site.BaseURL
- Menu URLs must include language prefix when defaultContentLanguageInSubdir = true
- Added translationKey to frontmatter for proper translation discovery

Documented 5 critical bugs and their solutions in design.md for future reference.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

content/_index.de.md

@@ -0,0 +1,17 @@
+---
+description: "Offene, ehrliche und nach Sinnhaftigkeit strebende Entwickler-Persönlichkeit."
+---
+
+<div class="row">
+  <div class="col-md-2">
+    <img src="/images/profile/markusgraf.jpg" alt="Portraitbild Markus Graf" class="img-fluid" />
+  </div>
+   <div class="col-md-10">
+
+## Hey, ich bin Markus
+
+- 🧘‍♂️ Offene, ehrliche und nach Sinnhaftigkeit strebende Entwickler-Persönlichkeit.
+- 🔬 Erfahrungen im wissenschaftlichen Arbeiten und in der Datenanalyse.
+- 👨‍💻 Über 20 Jahre Erfahrung in der Konzeption, Entwicklung und Wartung von Software.
+  </div>
+</div>