the API page needs an update wrt centres listing

[bansp]

Oopsie: now, with the centres.xml gone and redistributed among the individual recommendations, we need an update of the API page at https://standards.clarin.eu/sis/views/api.xq (see the last section).

[bansp]

BUT... maybe we could turn the tables a bit on this one. Why don't we generate centres.xml by script from the recommendations, in time adding to that the functionality from #242 (a check for whether the links are live, etc.).

In that way, the API and its documentation could remain as they are, and the script would act as yet another consistency check.

[bansp]

We're postponing the script issue and the main issue is solved, so closing.