document the doc generation process

[Rich-Harris]

It's a little hard to reverse engineer at the moment — I got very confused trying to figure out how stuff worked. A few things we could do to make life easier for future us:

• add more comments inside the script itself
• update apps/svelte.dev/README.md to describe the process
• add comments to the top of files that get copied over from repos
• when using the snippet cache to render a page, say so in the console
• (stretch goal) find the most recently updated file in the set of modules that collectively define the snippet generation process, and use its mtime in the snippet cache key

[Rich-Harris]

Yikes, I just saw a production deploy with a stale snippet cache. That's very bad. I think we probably need to do snippet generation at sync time

[dummdidumm]

...or do the more elaborate snippet cache key, or just disable it entirely for production deployments.