Add option to export Rd documentation

### Please describe your feature request

I think the situation now is that `box::help()` uses ` box:::parse_documentation()` to create `Rd` files to show help. In a situation where the documentation needs to be rendered into a web page it would be nice to have the Rd files to make `qmd`/`html` out of it.

I am asking for this in relation to the [r-project-template](https://github.com/novica/r-project-template) I am building that uses `box` (and other tooling in a non `R` standard way). And the current documentation workflow uses [quartify](https://github.com/ddotta/quartify) but maybe it would be useful to drop that dependency, make the `Rd` files with `box` and then pass them to [rd2qmd](https://github.com/eitsupi/rd2qmd) for `quarto` conversion in a CI setup to generate a web page documenting the project.

I can try making a PR if this is something you would approve for the package.

Start by reading box::help() and the internal box:::parse_documentation() path described in the issue. Determine where an export option could expose the generated Rd files without changing normal help behavior. Done means callers can obtain Rd files for downstream qmd or HTML conversion, with coverage for the new option if the project’s existing tests provide a suitable location.

Written by the indexing model from the issue text.