Add crosswalk page for SPDX 2.3 (#61)

The crosswalk CSV is at https://github.com/codemeta/codemeta/blob/master/crosswalks/SPDX_2_3.csv

Author: bact

content/crosswalk/spdx2_3.md

@@ -0,0 +1,22 @@
+---
+title: "Crosswalk for SPDX 2.3 SBOM"
+image: "/img/spdx.png"
+date: "2024-01-15"
+---
+
+The [Software Package Data Exchange® (SPDX®)](https://spdx.dev/) specification defines a standard data format for communicating the component and metadata information associated with software packages, widely known as "software bill of materials" or "SBOM". An SPDX document can be associated with a set of software packages, files or snippets.
+
+SPDX 2.3 supports multiple serialization formats and often uses the following file extensions:
+
+| Serialization format | File extension(s)              |
+|----------------------|--------------------------------|
+| tag:value            | `*.spdx`                       |
+| JSON                 | `*.spdx.json`                  |
+| RDF (RDF/XML)        | `*.spdx.rdf`, `*.spdx.rdf.xml` |
+| XLS spreadsheet      | `*.spdx.xls`, `*.spdx.xlsx`    |
+| XML                  | `*.spdx.xml`                   |
+| YAML 1.2             | `*.spdx.yaml`, `*.spdx.yml`    |
+
+The crosswalk for the SPDX 2.3 SBOM is as follows:
+
+{{< crosswalk name="SPDX_2_3" >}}