feat: add Vuex section

[afontcu]

• showcase how to test a component that interacts with a store.
• showcase how to mock/stub some potentially expensive Vuex actions (through jest.mock).
• explain how, if needed, testing a Vuex store in isolation is no different from testing any other JS function/object.

I'm assuming Vuex 4, by the way.

[lmiller1990]

You can use this for some inspiration: https://github.com/vuejs/vue-test-utils-next/blob/master/tests/features/vuex.spec.ts

I think it would be good to include both mocking the store, and using a real store, and the pros/cons.

[lmiller1990]

I will work on this one now

Writing docs is harder than writing code