German-language deep-dive guides for DIN/EN calculation topics

[YurMil]

The site UI is available in German but all content is English. Write 3–4 German-language docs/blog articles for the topics where German-speaking engineers are the primary audience: DIN 28011/28013 dished ends, EN 13445 blind flange sizing, EN 837 pressure gauge selection.

Uses the existing Docusaurus i18n content mechanism (i18n/de/docusaurus-plugin-content-*/).

Why: the DIN/EN standards the tools implement are native to the German-speaking market; German-language explanations serve that audience far better than translated UI alone.

Acceptance criteria:

• 3+ articles published under the de locale
• Each links to the relevant calculator
• Terminology reviewed against the German originals of the standards

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