Run as_gt() examples conditionally

[nanxstats]

Fixes #334

I used @examplesIf to only run the as_gt() examples when it is either in interactive mode OR not rendered by pkgdown.

#' @examplesIf interactive() && !identical(Sys.getenv("IN_PKGDOWN"), "true")

This will make the examples section in the pkgdown site not showing the 7,000 lines of raw HTML.

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I did not choose to capture the tables as images and create an "example output" section to link them (what gtsummary does) because:

• gt outputs is not the main theme of this package so it's less important to show what the output looks like.
• Since we have 11 tables here, the saved PNG files will bloat up the bundled package size by at least 1 Mb.
• One needs to regenerate the images using a script if the examples are updated. As people tend to forget about things outside of the standard workflow, it could become easily out of sync or a maintenance burden.

[LittleBeannie]

Thanks, @nanxstats !