Have you read the Contributing Guidelines on issues?

• I have read the Contributing Guidelines on issues.

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

The docs say that adding format: md front matter makes the doc to be CommonMark format, but it doesn't work.
After looking into the code, it should be

---
mdx:
  format: md
---

It reads the mdx in front matter first

But the docs say it's format in front matter.

Should it be fixed in the code or in the docs?

Reproducible demo

No response

Steps to reproduce

1. add format: md front matter in .mdx file
2. nothing happened

Expected behavior

Doc changes to CommonMark syntax.

Actual behavior

Doc stay with JSX syntax

Your environment

• Public source code:
• Public site URL:
• Docusaurus version used: v3
• Environment name and version (e.g. Chrome 89, Node.js 16.4):
• Operating system and version (e.g. Ubuntu 20.04.2 LTS):

Self-service

• I'd be willing to fix this bug myself.