docs: Migrate Preact docs from `wmr` to `create-preact`

[rschristian]

Summary

wmr is unfortunately no longer maintained. It was largely extracted into Vite and it's what we (the Preact team) recommend using in its place, via create-preact.

Prerendering is still functionality that's available to users, but requires a bit more work to setup than just a flag, so I moved the note on prerendering up a bit as it's an option users can enable when initializing a project. If that's what a user wants, easier to have us set it up for them than reading the docs to figure out which bits they need.

Screenshots (optional)

N/A

Documentation checklist

• [X] The documentation style guide has been adhered to.