DISCOVERY: (Restruct) Merge 🧩 `/concepts` with 📚 `/reference`

[azinit]

Steps

• [ ] Swap specific articles
• [ ] Merge/Split specific articles
• [ ] Update redirects
• [ ] Update reference links (Index-page, instead of concrete pages)
  • [ ] Remove old cross-linking to sections
• [ ] Update both locales (ru, en)
• [ ] Check cross-pagination
• [ ] Reorder some items if needed (e.g. - concepts by difficulty)
• [ ] Group some items if needed (promo & history & mission ...)
• [ ] (Maybe) Add old decompose view for compatability
• [ ] Add redirect page map (for old users)

[azinit]