docs: add exporter docs

This patch adds more complete documentation for the various exporters
available to buildx.

Co-Authored-By: David Karlsson <david.karlsson@docker.com>
Signed-off-by: Justin Chadwell <me@jedevc.com>

docs/guides/exporters/local-tar.md

@@ -0,0 +1,31 @@
+# Local and tar exporters
+
+The `local` and `tar` exporters output the root filesystem of the build result
+into a local directory. They're useful for producing artifacts that aren't
+container images.
+
+- `local` exports files and directories.
+- `tar` exports the same, but bundles the export into a tarball.
+
+## Synopsis
+
+Build a container image using the `local` exporter:
+
+```console
+$ docker buildx build --output type=local[,parameters] .
+```
+
+```console
+$ docker buildx build --output type=tar[,parameters] .
+```
+
+The following table describes the available parameters:
+
+| Parameter | Value  | Default | Description           |
+| --------- | ------ | ------- | --------------------- |
+| `dest`    | String |         | Path to copy files to |
+
+## Further reading
+
+For more information on the `local` or `tar` exporters, see the
+[BuildKit README](https://github.com/moby/buildkit/blob/master/README.md#local-directory).