docs: Add coding agent skills demo video to getting started pages (#5237)

* docs: Add coding agent skills demo video to getting started pages

Add Loom demo video embed showing how to build CrewAI agents and flows
using coding agent skills. Added to introduction, quickstart, and
installation pages across all languages (en, ko, pt-BR, ar).

* docs: update coding skills description with install instructions

Replace demo description text with actionable install copy across
all languages (en, ko, pt-BR, ar) in introduction, quickstart, and
installation pages.