docs: reorganize tooljet tutorial images + add dashboard/modal screenshots

Respond to review feedback requesting a dashboard/modal screenshot, and
clean up the image layout for future maintenance.

- Add 4 new screenshots captured against a live ToolJet + Capella setup:
  * capella/data-api-enabled.png — Connect → Data API page with endpoint URL
  * setup/marketplace-install.png — Couchbase plugin card in Marketplace
  * dashboard/airlines-table-populated.png — Table bound to listAirlines
  * dashboard/airline-details-modal.png — Modal showing getAirline result
- Move 8 existing screenshots into images/{setup,operations}/ subfolders
  with descriptive names (e.g. get-doc.png -> operations/get-document.png)
- Update all image references in the tutorial markdown accordingly

Author: teetangh