feat(docs): Add documentation for signed document metadata

[saibatizoku]

Description

Moves documentation for Signed Document Metadata from catalyst-voices.

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 | ${\color{lightgreen}Pass: 228/228}$ | ${\color{red}Fail: 0/228}$ |

[dt-iohk]

@saibatizoku Looks good to me, the documentation is well explained and makes sense.

What do you think about adding a sample CBOR for an encoded document?

[saibatizoku]

@saibatizoku Looks good to me, the documentation is well explained and makes sense.

What do you think about adding a sample CBOR for an encoded document?

@dtscalac sounds good to me. I've added an issue for this, so that we can define and build a properly documented example.