🐛 Bug Report

Prerequisites

• I'm using the latest version of Docusaurus.
• I have tried the npm run clear or yarn clear command.
• I have tried rm -rf node_modules yarn.lock package-lock.json and re-installing packages.
• I have tried creating a repro with https://new.docusaurus.io
• I have read the console error message carefully (if applicable)

Description

Docusaurus can't be installed alongside React 17.

Steps to reproduce

Try to install this with pnpm:

  "dependencies": {
    "@docusaurus/core": "2.0.0-beta.8",
    "@docusaurus/preset-classic": "2.0.0-beta.8",
    "@docusaurus/theme-classic": "2.0.0-beta.8",
    "react": "17.0.2",
    "react-dom": "17.0.2"
  },

Expected behavior

Packages should install.

Actual behavior

It seems some deep dependency of Docusaurus has a strict React 16 requirement:

 ERR_PNPM_INVALID_PEER_DEPENDENCY  ..\..\packages\cloudvision-docs > @docusaurus/core > @docusaurus/utils: @mdx-js/runtime@1.6.22 requires a peer of react@^16.13.1 but version 17.0.2 was installed.

Your environment

• Docusaurus version used: 2.0.0-beta.8
• Environment name and version (e.g. Chrome 78.0.3904.108, Node.js 10.17.0): Node 14.15.5
• Operating system and version (e.g. Ubuntu 20.04.2 LTS): Windows 10