Don't link to mixOmics

[DavisVaughan]

@topepo can you re-document for me? I'm having a lot of trouble getting the documentation to render because I can't seem to find the right combination of packages. I think this .Rmd needs to be rendered into .md form, then into .Rd

If we link to mixOmics, this causes a Note about mixOmics not being available for cross referencing when parsnip is checked as a revdep of any package that it depends on, like hardhat. That forces a manual inspection by CRAN which is extremely annoying.

i.e.

package hardhat_1.1.0.tar.gz has been auto-processed. The auto-check found problems when checking the first order strong reverse dependencies.

Changes to worse in reverse depends:

Package: parsnip
Check: Rd cross-references
New result: NOTE
  Package unavailable to check Rd xrefs: ‘mixOmics’

[github-actions]

This pull request has been automatically locked. If you believe you have found a related problem, please file a new issue (with a reprex: https://reprex.tidyverse.org) and link to this issue.