Swap to roxygen2 v8.0.0

[VisruthSK]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

Migrate to latest Roxygen2 version and did a quick redoc.

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):

Visruth Srimath Kandali

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[VisruthSK]

Ok I looked closer into how we're documenting things and actually using R6 documentation is a much bigger hurdle than I thought. Will not be trying to swap to R6 docs in this PR--don't know if there's a massive benefit to changing either.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.21%. Comparing base (48b825d) to head (75e2504).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1185   +/-   ##
=======================================
  Coverage   91.21%   91.21%           
=======================================
  Files          15       15           
  Lines        6121     6121           
=======================================
  Hits         5583     5583           
  Misses        538      538           

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[jgabry]

Yeah because of the severe limitations of R6 roxygen documentation when we first created cmdstanr, we had to come up with a different way of doing the doc. I had tried using the roxygen R6 support at the time and it just wasn't good enough to make useful documentation. I haven't looked too closely into the new R6 doc functionality in roxygen2 yet, but just because I knew it would be a pain to update.

[jgabry]

Thanks, looks good, I just reran the one failing windows run.