Add vector-db entry for pgvector

[estherbester]

Review checklist

Don't worry about satisfying all items, it's fine to open a (draft) PR.

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

[vercel]

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[filopedraz]

Great, thanks. Please rebase with main. We fixed the build pipeline.