docs: Add link to Docker guide in prune --docker flag section (#12401)
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### Description
The `--docker` flag section in the [prune reference
docs](/docs/reference/prune#options) describes the Docker-optimized
output format but doesn't link to the Turborepo [Docker
guide](/docs/guides/tools/docker), which contains an example Dockerfile
demonstrating how to use it.
This adds a sentence linking to the Docker guide so users can easily
find the example Dockerfile.
### Testing Instructions
Visit the prune reference page at `/docs/reference/prune` and verify the
`--docker` section now includes a link to the Docker guide at
`/docs/guides/tools/docker`.
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