docs: update extension dev guide for plugin-based single-module layout

Rewrites the guide to use the BTrace Gradle Extension Plugin with a single module and dual source sets (api, impl).

Covers plugin DSL (id/services/shadedPackages/permissions), auto-generated manifest/properties, and shaded impl artifacts.

Removes obsolete two-module setup, manual manifest edits, and ad-hoc shadow config in favor of plugin defaults.

Adds distribution/installation notes for the generated API/Impl artifacts and ZIP.

Author: jbachorik