Enable a "dev" mode to generate docs for all dependencies like in rust

[mpadge]

RTD would be an ideal system to enable generation of "dev mode" documentation like in rust, where the .Rd databases of all (installed) dependencies are extracted and converted to items in a single, locally-generated site. That would be one unique and convincing demonstration of what this package can offer beyond more conventional, package-based documentation systems in R.

[mpadge]

PR #10 adds the base functionality.

TODO

• [ ] Parse .Rd \concept tags to group functions Also not really possible within the structure of .rst tocs, so not worth doing (at the moment)
• [x] Create full package documentation, including landing page from DESCRIPTION file, and vignettes
• [x] Clean output of Rd2md::Rd2markdown() to remove all automatically-inserted hyperlinks, which are interpreted as local MyST link targets and trigger warnings from sphinx
• [ ] Add title string for each function to package index page - looks terrible, and puts full strings in toc panel, so a no-go