chore(rust/signed-doc): Proper DocumentRefs functional validation

[Mr-Leshiy]

Description

Refactored the current implementation of functional validation related to the DocumentRefs.
Added a proper doc_refs_check reusable function, which contain all DocumentRefs necessary validation.

Related Issue(s)

Part of #499

Please confirm the following checks

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream module

[github-actions]

✅ Test Report | ${\color{lightgreen}Pass: 410/410}$ | ${\color{red}Fail: 0/410}$ |