[DOC-FIX] switch from Rd to markdown documentation

[yitao-li]

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Willingness to contribute

The MLflow Community encourages documentation fix contributions. Would you or another member of your organization be willing to contribute a fix for this documentation issue to the MLflow code base?

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URL(s) with the issue:

As suggested by Lorenz in yitao-li#1

Description of proposal (what needs changing):

Provide a clear description. Why is the proposed documentation better?

Markdown syntax is more lightweight compared to Rd

[lorenzwalthert]

I'd suggest to use https://github.com/r-lib/roxygen2md or usethis::use_roxygen_md()and compare the Rd diff. Markdown syntax is simpler than the Rd syntax, e.g. for cross-linking and many people are already familiar with it.

[dmatrix]

@lorenzwalthert Thanks for responding. Presumably, you'll review @yitao-li PR once filed.