fix(rust/signed-doc): Modify the DocumentRef structure

[bkioshn]

Description

Modify the DocumentRef structure according to https://input-output-hk.github.io/catalyst-libs/architecture/08_concepts/signed_doc/metadata/#document-reference

Related Issue(s)

#323

Description of Changes

Change the structure of DocumentRef from {id, ver} to DocumentRefs
that store a list of DocumentRef {id, ver, locator}

Breaking Changes

The structure of document ref changed

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