Convert roxygen2 comments to markdown

[SaranjeetKaur]

Turn on Markdown support for the healthgpsrvis package :

1. Since we’re updating an existing package, running usethis::use_roxygen_md() will modify the DESCRIPTION (to include Roxygen: [list](https://rdrr.io/r/base/list.html)(markdown = TRUE)) and prompt us to use the roxygen2md package to convert the existing docs.
2. Once the package roxygen2md is downloaded, run roxygen2md::roxygen2md() to convert existing Rd comments to markdown.
3. Run devtools::document() when you're done.
4. Finally run devtools::check().