Enhanced R help HTML pages

[noamross]

The R help system has capabilities for rich HTML content, dynamic content generation wiht Sexpr{}, and a mechanism for extensions via macro, but these are rarely used. Yet the help pages pages are the most easily reached documentation for most users and have the potential to convey much more information.

It would be great to have a package/framework to make it easier for authors to produce richer help pages. This might include:

• Embedding badges in pkg-level man files
• Inserting footers with links to GH repos, project/author web pages
• Creating links to vignettes and package indices from help pages
• Making it easier to insert images, plots, and htmlwidgets. (I found it a remarkable pain to get one image into the viridis help pages, and have managed to get a DataTable into a help page with quite a bit of hackery).
• knitting example outputs into the help page
• Appropriate fall-backs and compatibility for help pages converted to text, PDF, and pkgdown pages.
• Roxygen tags/extensions to use the above
• Tutorials and templates
• A project to implement some of this (e.g. footer links) across ROpenSci packages

This was (re)-inspired by this Twitter convo with @leeper about embedding testing and coverage info in R help pages. I've done a bit of experimentation, and the btts package is a neat experiment in HTML "overlays" on R help.

[noamross]

Related issues: #58, #44, #41