fix: update links to point at the Vuex 4 documentation (#671)

Author: skirtles-code

src/.vuepress/config.js

@@ -344,7 +344,7 @@ module.exports = {
               },
               {
                 text: 'Vuex',
-                link: 'https://vuex.vuejs.org/'
+                link: 'https://next.vuex.vuejs.org/'
               },
               {
                 text: 'Vue CLI',

src/guide/state-management.md

@@ -2,7 +2,7 @@
 
 ## Official Flux-Like Implementation
 
-Large applications can often grow in complexity, due to multiple pieces of state scattered across many components and the interactions between them. To solve this problem, Vue offers [vuex](https://github.com/vuejs/vuex), our own Elm-inspired state management library. It even integrates into [vue-devtools](https://github.com/vuejs/vue-devtools), providing zero-setup access to [time travel debugging](https://raw.githubusercontent.com/vuejs/vue-devtools/master/media/demo.gif).
+Large applications can often grow in complexity, due to multiple pieces of state scattered across many components and the interactions between them. To solve this problem, Vue offers [Vuex](https://next.vuex.vuejs.org/), our own Elm-inspired state management library. It even integrates into [vue-devtools](https://github.com/vuejs/vue-devtools), providing zero-setup access to [time travel debugging](https://raw.githubusercontent.com/vuejs/vue-devtools/master/media/demo.gif).
 
 ### Information for React Developers
 
@@ -118,4 +118,4 @@ You should never replace the original state object in your actions - the compone
 
 As we continue developing the convention, where components are never allowed to directly mutate state that belongs to a store but should instead dispatch events that notify the store to perform actions, we eventually arrive at the [Flux](https://facebook.github.io/flux/) architecture. The benefit of this convention is we can record all state mutations happening to the store and implement advanced debugging helpers such as mutation logs, snapshots, and history re-rolls / time travel.
 
-This brings us full circle back to [Vuex](https://github.com/vuejs/vuex), so if you've read this far it's probably time to try it out!
+This brings us full circle back to [Vuex](https://next.vuex.vuejs.org/), so if you've read this far it's probably time to try it out!

src/style-guide/README.md

@@ -1615,7 +1615,7 @@ app.component('TodoItem', {
 
 ### Non-flux state management <sup data-p="d">use with caution</sup>
 
-**[Vuex](https://github.com/vuejs/vuex) should be preferred for global state management, instead of `this.$root` or a global event bus.**
+**[Vuex](https://next.vuex.vuejs.org/) should be preferred for global state management, instead of `this.$root` or a global event bus.**
 
 Managing state on `this.$root` and/or using a global event bus can be convenient for very simple cases, but it is not appropriate for most applications.