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
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## Current Behavior
<!-- This is the behavior we have today -->
The docs specify that the react library generator supports using rspack
as a bundler option.
The react library generator does not have rspack as an option.
## Expected Behavior
<!-- This is the behavior we should expect with the changes in this PR
-->
Remove misleading docs
## Related Issue(s)
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merged. -->
Fixes #
