Improve documentation for BOM-Link externalReferences.type

[prabhu]

Currently, externalReferences supports both URL and BOM-Link. There are some types that are better expressed with BOM-Link and therefore must be preferred over a URL.

Below are some types:

• bom
• release-notes
• model-card
• formulation
• attestation
• vulnerability-assertion
• pentest-report

To start with we can improve the documentation and create use-case examples to better illustrate the use of CycloneDX for these types.