766d1b32e0
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