[all] Consider replacing DEV_MODE links to lit.dev with shortened/stable redirectable links

[kevinpschaaf]

As we're starting to include deep links to the lit.dev docs site from DEV_MODE warnings, it seems likely that we're bound to break these links in the future if we're not careful (since user-pinned code versions can live forever while lit.dev content is bound to evolve over time).

I propose we replace deep links embedded in code with a shortened URL scheme which can be easily redirected on the lit.dev site without worrying about how URLs on the site may evolve in the future. This would also have the side-benefit of having a central place on the lit.dev site that catalogs all pages that are referred to via code.

Example:

lit.dev/msg/render-method-removed -> https://lit.dev/docs/releases/upgrade/#litelement
lit.dev/msg/multiple-versions-loaded -> https://lit.dev/docs/tools/adding-lit/#multiple-versions
...

[kevinpschaaf]

Redirect manifest already exists here: https://github.com/lit/lit.dev/blob/main/packages/lit-dev-server/src/redirects.ts. Just need to scrub the links in code and make short-links for them.