Overhaul "Learn Shiny" content to be Express focused

[cpsievert]

To preview locally (with the shinylive extension using the latest versions of packages):

# First, make sure you can build shinylive
git clone https://github.com/posit-dev/shinylive
cd shinylive
make submodules
make all

# Link shinylive CLI to freshly built assets
pip install shinylive
shinylive assets link-from-local ./build

# Re-build with latest dev versions
cd packages
cd py-htmltools && git pull && cd ..
cd py-shiny && git pull && cd ..
cd py-shinywidgets && git pull && cd ..
cd py-faicons && git pull && cd ..
cd ..
make clean && make all

# Finally, build py-shiny-site from this branch
git clone https://github.com/posit-dev/py-shiny-site
cd py-shiny-site
git checkout learn-shiny-overhaul
make serve

[gshotwell]

Got up to the ui sections. I think this is great, but the reactive programming stuff still seems like it's not quite right for people learning about reactivity for the first time. I think we should ship them because they're still a big improvement, but we may want to revisit them again fairly soon.

[gshotwell]

Looks great, one thing is that quite a lot of sentences start with some version of "it's often useful" or "it's sometimes useful". I think we can more or less get rid of all of these and just jump right into the content. I made some suggested changes, but flagged a few of them just to be revised. I think it's a good idea to use second person language in these cases "input.select is for when you want to select an input"

[gshotwell]

Do we want to say more about this?

[cpsievert]

Ahh sorry, this was another file that should've been removed (we can lean on the layout gallery instead now).

[gshotwell]

I'm sorry to say I really don't like the direction that the Motivate express article has gone in. My main concern with Express is that we want to avoid abandoning Shiny Core, and so we need a very clear article which articulates why people should still care about and use Shiny Core. Currently the Express-Core translation article reads more like "Here's why you might think you need Shiny Core but really Express does all of those things too", and includes much more express code than Core code. My suggestion is:

• Write a new article called "programming with express" that covers @expressify @hold @render.express and talks about managing complexity in Express.
• Reference that article in a callout, but have this article be all about the virtues of Core.

If we don't have a clear articulation of why Core is good, Express looks a lot more like something that supersedes Core entirely than adjuvant syntax.