Better document vue/vuetify installation

[subdavis]

Made both Vue and Vuetify peer dependencies.

Had to change the install guide to explicitly install vuetify, and realized I was just replicating Vuetify docs inline.

So I switched to recommending vue add vuetify as the install mechanism, since it does quite a lot of work that a user would otherwise have to guess from looking at the GWC package json and vue.config.js

This way, for the most common case of "fresh project", you get a more first-class vuetify experience rather than trying to replicate our config by hand.

[subdavis]

The issue is installing vue/vuetify for running tests and building demo app.

Doing this is unsupported by yarn. After 4 years of debate, a maintainer has decided "you can do it with a plugin in yarn 2". There are weird packages to hack around it, but I don't feel good about using one.

The proper yarn 1.X solution would be either a) a monorepo or b) an entirely separate repo for the demo app, neither of which is appropriate for the level of effort involved.

I think my docs changes are still good.

PTAL.

[subdavis]

@zachmullen this is the last thing blocking the 3.0 chain of events. Mostly just because I want the NPM page to include these changes.