docs: complete user manual v1.0 (without screenshots)

docs/DOCUMENTATION_GUIDELINES.md

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+# Documentation Guidelines
+
+## Language
+
+The primary documentation language is German.
+
+An English version will be maintained as a translation of the German master document.
+
+## UI terminology
+
+All user interface terms remain in English.
+
+Examples:
+
+- Direct Calculation
+- Extrapolation
+- Roll Diameter
+- Product Length
+- Capacity Summary
+- Capacity Analysis
+- Governing Constraint
+
+Do not translate UI labels.
+
+## Style
+
+- Technical user manual
+- Workflow-oriented
+- Explain why, not only how
+- Engineering language
+- No marketing language
+
+## Workflow
+
+1. Write German chapter.
+2. Technical review.
+3. Add screenshots.
+4. Approve.
+5. Translate to English.