docs: update vue-router documentation

[blake-newman]

Update documentation to explain differences between using localVue and mocks and provide examples of the use case for each.

A common gotcha with integration testing (which don't stub children) is when using the mocks approach the router instance is not available on child components

[jamiejones93]

🎉

[afontcu]

Neat!

Looks like some formatting is a bit off:

Checking formatting...
docs/guides/using-with-vue-router.md
Code style issues found in the above file(s). Forgot to run Prettier?

Would you mind running Prettier and push again? (We should def ease this process with a precommit hook or similar…)

[blake-newman]

@afontcu or i was lazy and committed via github 😆

[lmiller1990]

I never realized mocks was only for the component in question - I thought it would be available to all the children, too 🤔

It sounds like this should be the default behavior.

[blake-newman]

@lmiller1990 I have created a new issue. Still believe best to document the current behaviour.