Adds Pro trial action button for welcome webview

Introduces a new "Start GitLens Pro Trial" button to the welcome webview and tracks user interactions with a dedicated telemetry event. Updates telemetry documentation and types to include "welcome" as a source and to describe the new "welcome/action" event, enabling better insight into user engagement with onboarding and trial features.
(#4769, #4773, PLG-138)

Author: sergeibbb