docs: upgrade vuepress (#2296)

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- For: vuejs/vuepress#645, 
- Preview: https://deploy-preview-2296--vue-router.netlify.com/

Author: Luisa-m17

docs/.vuepress/override.styl renamed to docs/.vuepress/style.styl

@@ -74,7 +74,7 @@
     "vue": "^2.5.16",
     "vue-loader": "^15.2.1",
     "vue-template-compiler": "^2.5.16",
-    "vuepress": "^0.10.2",
+    "vuepress": "^0.12.0",
     "vuepress-theme-vue": "^1.0.3",
     "webpack": "^4.9.0",
     "webpack-dev-middleware": "^3.1.3"