Documentation: reframe "Publishing" section in terms of use cases

[angela-li]

The "Publishing" section of the pkgdown website would be more useful to users if it was framed in terms of the needs of the users of learnr tutorials. We (@mine-cetinkaya-rundel @jules32 @isteves and I) thought of three users:

1. Instructor who sends students to a Shiny web link to run the tutorial through their browser (perhaps, using the free academic version of Shiny Server)
2. Instructor who wants students to install a tutorials package, and do run_tutorial() locally
3. Package developer who wants to include a learnr tutorial in their package to demonstrate how to use it

The recommended publishing route in each of these cases would then be:

1. Deploy on shinyapps.io / Shiny Server
2. / 3. Include one or multiple learnr tutorials in an R package

We are thus proposing replacing in the Publishing section:

The most straightforward way to deploy a tutorial is to include it within an R package and have users run it directly via the learnr::run_tutorial function.

With something like:

"If you would like to provide a browser link for students to use your learnr tutorial without further configuration on their end, do X. If you would like students to download a package so that they can run tutorials locally, do Y. If you want to include a learnr tutorial in your existing package to teach users how to use it, do Z."

[schloerke]

I'll gladly take a PR 😄