Add docs for `<NuxtPage>` component

Editor assessment

The subject names the exact component documented and places the change squarely in docs(api), so a reader knows what was added without opening the diff. The imperative add and clean punctuation keep the header tidy, and no footer is needed for documentation work. The absence of a body is acceptable for a focused docs addition, though one sentence about what the `<NuxtPage>` documentation covers would have added value. All five dimensions are served without error; only the missing rationale keeps this from the top band, leaving a clear, publishable example.