[Docs] Velocity Playground: document UI/UX changes after Dojo → Angular migration

[hmoreras]

Goal

Document the UI/UX changes from the Velocity Playground Dojo → Angular migration. Delivered when the FE PR is merged.

Scope

• Update the dotCMS user docs page for Velocity Playground with new screenshots and any behavioral changes called out in the parity matrix (from the consolidated spike) — e.g. Ace → Monaco editor, history affordance, output content-type rendering.
• Add a short developer note describing the rollback toggle (velocity_playground vs velocity_playground-legacy) and pointing to the new Angular portlet path (core-web/libs/portlets/dot-velocity-playground/).

Definition of Done

• User docs page updated and published.
• Developer note added in the appropriate location in the repo or docs site.
• Cross-linked from epic Dojo to Angular: Velocity Playground Portlet #34737.