Roxygen docs

[fangzhou-xie]

This PR does the following:

1. Roxygen documentation for functions (J and translate)
2. Roxygen auto-generate NAMESPACE file

After merging your recent fix for the gamma-functions, the devtools::check() shows no error/warning/note with this PR.

Also, for the pkgdown website, please run the following commands:

usethis::use_github_action("pkgdown")
usethis::use_pkgdown_github_pages()

(The second command requires push access for the repo and it should generate a branch called gh-pages and you can then set up the Github pages in the settings of the repo. The first command will setup Github action to auto update the website after each merge to the main branch.

Thanks.