This PR updates the 4 markdown files in the Core Concepts / In-Depth Topics to use gjs.

I tried to limit the changes here to only migrating from js+hbs to gjs. In a few places, some reordering of the prose around the code snippets was unavoidable. Based on the number of changes, I'd recommend going through the rendered docs and see if anything looks awry.

Some notes:

• Since gjs requires explicit imports, I added imports for things like Input and modifier. But I don't know the imports for the component helper and helper helper, so those should be addressed.
• In autotracking-in-depth, some of the snippets now have diff indicators where they didn't before.
• In rendering-values, everything that was just handlebars has been wrapped in <template></template> in a gjs block. Many of these care incomplete however, because they reference stuff that is not in scope. I think this is fine, but maybe somebody else has an opinion on a better way to handle these.

Leaving as a draft until I figure out the imports.