docs: add learning resource 'Become a Ninja with Vue 3' (#898)

Shameless plug to link my ebook and online course as they have a large section about testing (and hundreds of unit tests examples) using VTU

Author: cexbrayat

docs/guide/extending-vtu/community-learning.md

@@ -1,6 +1,7 @@
 # Community and Learning
 
 - [Vue 3 Testing Handbook](https://lmiller1990.github.io/vue-testing-handbook/v3/). Online guide.
+- [Become a Ninja with Vue 3](https://books.ninja-squad.com/vue). Ebook and online course.
 - [Vue.js 3: The Composition API](https://vuejs-course.com/composition-api). Online course (includes a testing module).
 - [Testing Vue 3 apps with Vue Test Utils](https://www.youtube.com/playlist?list=PLC2LZCNWKL9ahK1IoODqYxKu5aA9T5IOA). Playlist.
 - [Testing Vue.js Components](https://vueschool.io/courses/learn-how-to-test-vuejs-components?friend=vth). Online course.