[docs] Align GitBook theming to main Vue.js site (#448)

* [docs] Align GitBook theming to main Vue.js site

* Revert "[docs] Align GitBook theming to main Vue.js site"

This reverts commit 9aad1473626f1cc9b5bc995cb4028c0412b2c3df.

* [docs] Move all css to a plugin

docs/book.json

@@ -1,6 +1,6 @@
 {
-  "gitbook": "2.x.x",
-  "plugins": ["edit-link", "prism", "-highlight", "github"],
+  "gitbook": ">3.0.0",
+  "plugins": ["edit-link", "prism", "theme-vuejs@git+https://github.com/pearofducks/gitbook-plugin-theme-vuejs.git", "-fontsettings", "-highlight", "github"],
   "pluginsConfig": {
     "edit-link": {
       "base": "https://github.com/vuejs/vuex/tree/dev/docs",