Bump @docusaurus/preset-classic in /docs (#3451)

Bumps [@docusaurus/preset-classic](https://github.com/facebook/docusaurus/tree/HEAD/packages/docusaurus-preset-classic) from 2.0.0-alpha.65 to 2.0.0-alpha.70.
- [Release notes](https://github.com/facebook/docusaurus/releases)
- [Changelog](https://github.com/facebook/docusaurus/blob/master/CHANGELOG-2.x.md)
- [Commits](https://github.com/facebook/docusaurus/commits/v2.0.0-alpha.70/packages/docusaurus-preset-classic)

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docs/package.json

@@ -13,7 +13,7 @@
   },
   "dependencies": {
     "@docusaurus/core": "^2.0.0-alpha.65",
-    "@docusaurus/preset-classic": "^2.0.0-alpha.58",
+    "@docusaurus/preset-classic": "^2.0.0-alpha.70",
     "clsx": "^1.1.1",
     "react": "^16.8.4",
     "react-dom": "^16.8.4"