Describe it-ticket exemption codes

[rilla]

Context

The it-ticket-exempt codes are published with bare labels — "Not subject", "Not taxable", "Exempt" — which don't say what distinguishes them. An integrator picking a code for a zero-VAT line can't tell from the reference which one applies, and the wrong choice is a misdeclaration even though the VAT is zero either way. This came up in support: a customer asked us to explain the difference between N2, N3 and N4 after reading the addon page.

it-sdi already carries descriptions on its extensions, and cbc.Definition.Desc exists for exactly this.

Changes

• Add a description to each of the six exemption codes, naming the kind of operation each one covers (excluded amounts, outside scope including the flat-rate regime, not taxable by law, exempt under Art. 10, margin scheme, reverse charge). Regenerated data/addons/it-ticket-v1.json, which is what docs.gobl.org renders.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.95%. Comparing base (0b22571) to head (3bbabec).

Additional details and impacted files

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  Coverage   94.95%   94.95%           
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  Files         327      327           
  Lines       17668    17668           
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  Hits        16777    16777           
  Misses        549      549           
  Partials      342      342           

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