docs: documentation for `--output` options is distributed

Description

docs: documentation for --output options is distributed

While writing up this comment https://github.com/moby/moby/issues/50834#issuecomment-3232592437 I noticed that the information about the --output flag is a bit distributed. I looked at the documentation for the flag (at https://docs.docker.com/reference/cli/docker/buildx/build/#output), then wanted to suggest that it's possible to specify the compression, but I couldn't find it on that page! "Search" in the docs didn't help immediately, but "Ask AI" did point me to the right page; https://docs.docker.com/build/exporters/#configuration-options.

That page is a bit in reverse; it has details on the compression, but is more "sparse" on the output types. We should probably make sure that at least the "compression" (and related) options are mentioned in the --output flag reference (but we could of course link to the other page for further details if we don't want to repeat).

I'm also wondering if the (subtle?) differences between the oci and docker outputs should get detailed a bit more; they describe that they're different (but the docker one also being OCI compliant), but don't really help me pick "which one should I use?"; i.e.

• do I lose information if I pick the docker type (will it discard the docker-specific additions to the spec)?
• if I use docker, are extra features, like attestations preserved, or are those only included in the oci type?
• same; do both maintain multi-platform images, or if I use oci?
• can both be loaded using docker image load ?

Start with the --output reference in the Docker buildx build documentation and compare it with the exporters configuration options page. Document compression and related options in the flag reference, and clarify the practical differences between oci and docker, including attestations, multi-platform images, information loss, and docker image load support.