JSON-LD Serialization doc: Add info on validation and Examples, sort use cases/examples list

[bact]

• Rename serialization/json_ld/ directory to serialization/json-ld/, for consistency with serialization/json-ld.md file
  • Note that in spdx-spec, we use jsonld - so there's still an inconsistency across repos/websites within the same proejct. See: https://github.com/spdx/spdx-spec/tree/development/v3.0.1/examples/jsonld
  • It will be nice to pick just one from: json_ld, json-ld, jsonld
• In serialization/json-ld.md:
  • Add info about validation section, link to serialization/json-ld/validation.md
  • Add Examples section and add link to serialization/json-ld/examples.md
• Sort use cases/serialization examples list