docs(rfc): add RFC for @org default templates in vp create

Specify a manifest-based extension to `vp create` so organizations can expose a
curated set of templates through a single entry point (`vp create @org`). Builds
on the existing `@org` → `@org/create` shorthand; adds a `vp.templates`
manifest in `@org/create`'s `package.json`, an interactive picker, a per-entry
`monorepo` flag mirroring the builtin `vite:monorepo` filtering, and a
`create.defaultTemplate` project config in `vite.config.ts`.

Author: fengmk2