WIP: EN16931 mappings to GOBL

[samlown]

Describe your changes

Checklist before requesting a review

• I have performed a self-review of my code.
• I have added thorough tests with at least 90% code coverage.
• I've modified or created example GOBL documents the show my changes in use, if appropriate.
• When adding or modifying a tax regime or addon, I've added links to the source of the changes either structured or in the comments.
• I've run go generate . to ensure that Schemas and Regime data are up to date.
• All linter warnings have been reviewed and fixed.
• I've been obsessive with pointer nil checks to avoid panics.
• Requested a review from @samlown.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 13 lines in your changes missing coverage. Please review.

Project coverage is 78.73%. Comparing base (15397b4) to head (95b124a).
Report is 199 commits behind head on main.

Files with missing lines	Patch %	Lines
tax/mapping_table.go	0.00%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #401      +/-   ##
==========================================
- Coverage   78.82%   78.73%   -0.10%     
==========================================
  Files         217      218       +1     
  Lines       10721    10734      +13     
==========================================
  Hits         8451     8451              
- Misses       1914     1927      +13     
  Partials      356      356              

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[samlown]

This has been moved to the documentation site: https://github.com/invopop/gobl.docs