As user, I don't want to have to think if I need `inside_rmd` or not when using `att_from_rmd()`

[statnmap]

As user, I don't want to have to think if I need inside_rmd or not when using att_from_rmd(), so that the same function can be presented in help as in the classical use.

Criteria of success

• There is no more inside_rmd parameter to be chosen by the user
• att_from_rmd() works the same, inside or outside rmd
• Readme and doc of {attachment} is simplified to deal with this change
• Make {callr} a Suggest of the package

Tech

• use outside_knit <- is.null(knitr::opts_knit$get("out.format")) to detect if we are in or outside a knitting environment, thus running directly the code, or run using callr:r() call.
• {callr} is only needed if using the function when running knit, which is not really frequent with {attachment}, except for documentation and courses.