chapter: models (update)

[biswaroop1547]

Review checklist

• [x] chapter content
  • [x] only one top-level # h1-Title
  • [x] summary (e.g. table or TL;DR overview), no need for an explicit ## Summary/Introduction title or equivalent
  • [x] main content focus: recent developments in open source AI
  • general context/background (brief)
  • current pros/cons
  • in-depth insights (not yet widely known)
  • [x] likely ## Future developments
  • [x] end with {{ comments }}
• [x] appropriate citations
  • [x] BibTeX references
  • [x] Glossary terms
  • [x] cross-references (figures/chapters)
  • [x] (if new-chapter.md), add _toc.yml entry & index.md table row
  • [x] If CI URL checks have false-positives, append to _config.yml:sphinx.config.linkcheck*
• [x] images & data not committed to this repo (e.g. use https://github.com/premAI-io/static.premai.io instead)