Update SBOM Document Quality Guide and externalize image assets

Signed-off-by: Kouki Hama <hamakouki0818@gmail.com>

Author: KoukiHama

Cross-Industry-SBOM-Quality-Guide/en/Cross-Industry-SBOM-Quality-Guide.md

@@ -10,4 +10,6 @@ Such a guide would:
 In this folder you will find a copy of the brainstorm document. 
 
 You can track the discussion via the OpenChain SBOM Study Group mailing list:
-https://lists.openchainproject.org/g/sbom
+https://lists.openchainproject.org/g/sbom
+
+Illustration files referenced by `Cross-Industry-SBOM-Quality-Guide.md` are stored under `assets/images/sbom-document-quality-guide/` so that text changes and binary asset changes can be reviewed separately in GitHub.