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

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."