Restore schemas directory for IANA compatibility

[bact]

The schemas directory is referenced in official specifications, including IANA media type registration.
It is also referenced in examples/README.md (alongside the ontology directory).

To fix #1158 (reported by @RingoDev), this PR:

• Restore schemas directory: Maintains directory structure as documented by IANA (application/spdx+json) and the SPDX v2.3 Specification (4.4 Standard data format requirements ).
• Keep spdx-schema.json pointing to the latest SPDX 2.x version, for backward compatibility. (SPDX 3 versions can access the schema at https://spdx.github.io/spdx-spec/VERSION/rdf/schema.json).
• Rename rdf directory back to ontology directory.
  • This will not break SPDX 3 publication workflow, as the RDF directory for publication is docs/rdf.

Notes: since IANA points to https://github.com/spdx/spdx-spec/blob/master/schemas/spdx-schema.json -- the change in this PR (develop branch) has to be mirrored in main branch too. GitHub will automatically redirected master branch URLs to main.

File	Source
spdx-schema-2-2.json	https://github.com/spdx/spdx-spec/blob/development/v2.2/schemas/spdx-schema.json
spdx-schema-2-2-1.json	https://github.com/spdx/spdx-spec/blob/development/v2.2.1/schemas/spdx-schema.json
spdx-schema-2-2-2.json	https://github.com/spdx/spdx-spec/blob/development/v2.2.2/schemas/spdx-schema.json
spdx-schema-2-3.json	https://github.com/spdx/spdx-spec/blob/support/2.3/schemas/spdx-schema.json
spdx-schema-2-3-1-dev.json	https://github.com/spdx/spdx-spec/blob/support/2.3.1/schemas/spdx-schema.json
spdx-schema-3-0-1.json	https://spdx.github.io/spdx-spec/v3.0.1/rdf/schema.json
spdx-schema-3-1-dev.json	https://spdx.github.io/spdx-spec/v3.1-dev/rdf/schema.json
spdx-schema.json	Latest 2.x (symlink)

[goneall]

I think this should be fine to add back for compatibility.

@zvr - any concerns?