Add yarn version of all commands

[rootwork]

e.g. yarn global add premail in addition to npm i -g premail.

First step is to install this plugin:
https://docusaurus.io/docs/next/markdown-features/code-blocks/#npm2yarn-remark-plugin

[rootwork]

See that PR from premail/premail for the syntax for the Yarn commands, which is slightly different. Yarn suggests against global installs but it is possible.