[Required] Improve security self-assessment doc

From CNCF TOC:

> KServe Security Self Assessment is available at https://github.com/kserve/community/blob/main/security/self-assessment.md.

**TODO for KServe Team:**

1. Link to the CI/CD generated SBOMs at https://github.com/kserve/community/blob/main/security/self-assessment.md#software-bill-of-materials
2. Clarify if the generated docker images are scanned for security vulnerabilities at https://github.com/kserve/community/blob/main/security/self-assessment.md#development-pipeline
3. CONTRIBUTING.md should also include information about how to report security vulnerabilties at https://github.com/kserve/community/blob/main/security/self-assessment.md#development-pipeline