docs: add index page for attestations

Signed-off-by: Justin Chadwell <[email protected]>

Author: jedevc

docs/attestations/README.md

@@ -0,0 +1,16 @@
+# Attestations
+
+BuildKit supports creating and attaching attestations to build artifacts.
+Generated attestations use the [in-toto attestation format](https://github.com/in-toto/attestation).
+
+The currently supported attestation types are:
+
+- [SBOMs](./sbom.md)
+- [SLSA Provenance](./slsa-provenance.md)
+
+Upon generation, attestations are attached differently to the export result:
+
+- For the `image`, `oci` and `docker` exporters, attestations are exported
+  using the attached [attestation storage](./attestation-storage.md).
+- For the `local` and `tar` exporters, attestations are written to separate
+  files within the output directory.