Add Angular 21 plugin migration guide

Comprehensive documentation covering all changes between stable (Angular 20)
and angular19 branch (Angular 21): standalone component conversion, NgModule
refactoring, bootstrap changes, package versions, TypeScript config, build
config, SCSS fixes, template ID indexing, and Jest to Vitest test migration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: renemadsen