sbom tool cycloneDX

[ErniGH]

conan-io/conan#17559

[memsharded]

It is looking good, a couple of small additions:

• Adding the conanfile.subgraph docs somewhere, as experimental too
• Adding a small example with a hook using the .subgraph.serialize() to serialize the conan graph.json. It can be done also to put it inside the binary package metadata folder in the same way as the cyclonedx. It can be a parallel section to the CycloneDX one, called Conan
• Remind somewhere that the contents of the files are also subject to modifications and evolution, they might change in future version