docs: document metadata pagination field (#95509)
### What?
Documents the `pagination` metadata field in the `generateMetadata` API
reference.
### Why?
The `Metadata` type supports `pagination.previous` and
`pagination.next`, but the Metadata Fields documentation did not list
the field.
Fixes #83264
### How?
Adds a `pagination` section with an example and the generated `<link
rel="prev">` / `<link rel="next">` output.
### Tests
- `pnpm prettier --check
docs/01-app/03-api-reference/04-functions/generate-metadata.mdx`
- `pnpm lint-eslint
docs/01-app/03-api-reference/04-functions/generate-metadata.mdx`
- `git diff --check HEAD~1..HEAD`
<!-- NEXT_JS_LLM_PR -->
(cherry picked from commit dc5fe22519e355cdf5189aa96e8e30af0a68d46e)
