docs(readme): use pnpm to manage node versions instead of nvm

## Problem Statement

Current README encourages the use of [nvm](https://github.com/nvm-sh/nvm).


This is unneccessary, as the project uses [pnpm](https://pnpm.io), which is already capable of managing node versions via `pnpm env use --global ...` since version 6.

## Proposed Solution

Explore pnpm to manage node versions instead of nvm, and update readme accordingly.

https://pnpm.io/cli/env


Start by locating the README section that currently recommends nvm, then review pnpm's env command documentation at https://pnpm.io/cli/env. Done means the README explains how to manage the project's Node.js version with pnpm instead of nvm and no longer directs users to the old workflow.

Written by the indexing model from the issue text.