docs(adr): Change Textual SignDoc to struct

[amaury1093]

Description

When discussing today about Spec updates, we came up with the argument that structs are more backwards-compatible than arrays. Propose to switch the sign doc to a struct containing one field (the old array).

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Author Checklist

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I have...

• included the correct type prefix in the PR title
• added ! to the type prefix if API or client breaking change
• targeted the correct branch (see PR Targeting)
• provided a link to the relevant issue or specification
• followed the guidelines for building modules
• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
• included comments for documenting Go code
• updated the relevant documentation or specification
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• confirmed the correct type prefix in the PR title
• confirmed ! in the type prefix if API or client breaking change
• confirmed all author checklist items have been addressed
• reviewed state machine logic
• reviewed API design and naming
• reviewed documentation is accurate
• reviewed tests and test coverage
• manually tested (if applicable)

[amaury1093]

All CBOR encodings now have a101 prepended. Looking at the RFC Appendix B, this makes sense:

 | 0xa0..0xb7 | map (0x00..0x17 pairs of data items follow)    |

• a1: map with 1 item
• 01: key of first item

[amaury1093]

I'll enable auto-merge once it's tested with Zondax's new ledger app

[amaury1093]

Tested, merging.