docs: upgrade to vuepress 1.x

package.json

@@ -44,6 +44,7 @@
   },
   "devDependencies": {
     "@vue/conventional-changelog": "^0.1.1",
+    "@vuepress/theme-vue": "1.0.0-alpha.44",
     "babel-core": "7.0.0-bridge.0",
     "babel-eslint": "^10.0.1",
     "conventional-changelog": "^3.0.6",
@@ -68,7 +69,7 @@
     "rimraf": "^2.6.2",
     "semver": "^5.5.0",
     "typescript": "^3.3.3333",
-    "vuepress": "^0.14.9",
+    "vuepress": "1.0.0-alpha.44",
     "vuepress-theme-vue": "^1.1.0",
     "webpack": ">=4 < 4.29",
     "yorkie": "^2.0.0"