Add end-user documentation for IPN generation and synonym system

[Copilot]

Part-DB's IPN generation and synonym system lacked user-facing documentation. These features enable automatic part numbering schemes and terminology customization, but users had no guidance on configuration or usage.

Documentation Added

• IPN Generation (docs/usage/ipn_generation.md, 306 lines)

  • Automatic IPN generation with category-based prefixes and sequential numbering
  • Configuration of regex patterns, separators, digit counts, and global prefixes
  • Integration with labels, barcodes, and EDA tools
  • Best practices for prefix design and numbering schemes
  • Environment variable overrides for Docker deployments

• Synonym System (docs/usage/synonyms.md, 346 lines)

  • Terminology customization for 16 element types (part, category, manufacturer, etc.)
  • Language-specific synonym configuration
  • Use cases for non-electronics contexts (libraries, manufacturing, retail, laboratories)
  • Translation mechanism using placeholder injection
  • Current limitations as experimental feature

Cross-References Updated

• Added IPN documentation link in docs/concepts.md Part section
• Added both features to main feature list in docs/index.md

Both documents follow existing Jekyll/Just-the-Docs patterns with proper navigation ordering and validated cross-references.

Original prompt

Write documentation for the end user, about how IPN generation and the synonym system work.

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