docs(opengraph-image): load assets at module scope to keep route static under Cache Components (#95246)
### What?
Document loading request-independent assets at module scope, and update
the OG image examples to follow it.
### Why?
Reading a local asset inside the component is async I/O during render,
so the route defers to request time. A top-level `await` at module scope
keeps the route prerendered without reaching for `use cache`. The
pattern previously only lived as an inline snippet in the OG image docs.
### How?
- Add an "Asynchronous I/O at module scope" section to the Cache
Components guide as the canonical explanation.
- Load fonts and local assets at module scope in `opengraph-image.mdx`
and `image-response.mdx`, and link both to the new section.
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Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>