Document printer compatibility recommendations for the Foomatic directory (GSoC 2026)

[gativarshney]

Summary

This PR adds comprehensive documentation for the printer compatibility recommendation system developed as part of Google Summer of Code 2026 with OpenPrinting.

The documentation covers the recommendation pipeline from Foomatic data processing and feature generation through similarity computation, evaluation, retraining, and frontend integration.

Documentation included

• Foomatic data formats

  • Printer and driver data structures
  • Recommendation artifacts and schemas
  • Client-facing recommendation shards
  • Build-time data and evaluation artifacts

• Pipeline architecture

  • End-to-end recommendation pipeline
  • Data processing and feature generation
  • Similarity computation and ranking
  • Build and deployment flow

• Recommendation quality and evaluation

  • Scoring methodology
  • Feature weighting and confidence thresholds
  • Evaluation methodology and measured results
  • Known limitations and scope of the evaluation

• Retraining and regeneration

  • Prerequisites and pipeline commands
  • Stage-by-stage regeneration workflow
  • Local development options
  • CI automation and regression checks

• Extending the recommendation UI

  • Printer page architecture
  • Recommendation UI contract
  • Filtering and recommendation signals/features

This PR is intentionally documentation-only and contains no application or implementation changes.

This is a documentation follow-up to #224, which introduced the printer compatibility recommendation system.