guidance.yaml: Update BT-01 mappings and examples

[duncandewhurst]

Closes #226

Wait until open-contracting-extensions/ocds_legalBasis_extension#11 is merged before merging.

The ELI format for EU directives is documented here: https://eur-lex.europa.eu/eli-register/eu_publications_office.html

[duncandewhurst]

@jpmckinney the build check on this PR seems to be stuck on 'Expected', even though all the checks on the commit were successful.

I tried to merge the branch locally and push to main (if I remember correctly, our practice in this repo was to commit to main without PRs) but it seems like main is now protected.

[jpmckinney]

main branches are pretty much always protected. I had to update the branch protection rule, because a workflow name changed.