[Doc] Preview for CLI Migration

[rtritto]

Description

Before Vike CLI, the vite preview command isn't supported (#2010 (comment)) and node dist/server/index.mjs command must be used.

Using vike preview by CLI Migration page, that command still not supported.

node dist/server/index.mjs doesn't need any CLI (vite or vike)

Alternatives for doc

1. Nothing is changed (no migration needed).
   Diffs to fix doc:

   - -    "preview": "vite preview"
   - +    "preview": "vike preview"
   +      "preview": "node dist/server/index.mjs"

2. (preferred) Vike CLI uses the preview command (that call node dist/server/index.mjs) and could do other things.
   The [@brillout/vite-plugin-server-entry] Cannot find server entry #2010 (comment) should be reverted.
   Diffs to fix doc:

   - -    "preview": "vite preview"
   + -    "preview": "node dist/server/index.mjs"
    +   "preview": "vike preview"

[brillout]

👍 We could do this: vikejs/vike-node#41.