docs(readme): incorrect and missing pnpm scripts

[Jay-Karia]

Related page

https://github.com/mui/material-ui/tree/next/docs#docs

Kind of issue

Missing information

Issue description

The steps to start the documentation website locally, there is a script given: pnpm start which is incorrect.
It should be pnpm dev.

Before starting the development server, we need to install required packages by pnpm install, this script is missing.

Context

No response

Search keywords: incorrect script, missing script