feat: Add Stubs/Shallow Mount section

[afontcu]

https://github.com/vuejs/vue-test-utils-next-docs/blob/master/src/guide/stubs-shallow-mount.md

[lmiller1990]

I think @dobromir-hristov was interested in writing this one.

@NataliaTepluhina also has a lot of knowledge on how/when to use these from her experience working on a huge Vue codebase at GitLab. We should definitely ask if she can review this section.

[dobromir-hristov]

Yep, give me a day or two :)

[lmiller1990]

No rush 🖖

[lmiller1990]

I would like to work on this one, I have some ideas.

Main points:

• stubs
• shallow: true
• the slot stuff
• "old" vs "new" behavior

Will ping for a review in a day or two

[afontcu]

cool! Are you planning to add some pro's and con's of using stubs and shallow mount? Similar to what Cypress does on Testing Strategies for Network Requests.

disclaimer: it might not be the place, and that content might live somewhere else! 😇

[lmiller1990]

Yes, I think we should include pros and cons. This section should be unopinionated, though. I'll get a few reviews from different people on this one, especially @dobromir-hristov and @NataliaTepluhina who have a lot more experience than me with stubs and shallowMount in very large applications 👍