This PR removes installation instructions for Nx < 18 since it has
fallen out of support window. Users can always `npm install <plugin>` as
usual if they are on an unsupported Nx version, but the docs will assume
`nx add` moving forward.
The changes are on the plugin overview page:
https://nx-dev-git-docs-remove-nx-pre-18-install-nrwl.vercel.app/nx-api