Document data availability queries

[khaeru]

#161 added support for these queries. One example is given in https://github.com/khaeru/sdmx/pull/161#issuecomment-1929361106, but these should also be documented, because of their (possibly) non-intuitive semantics: they use a data-like query and key, but return a StructureMessage with the AvailableConstraint and other artefacts.

Since there is no known public SDMX REST data sources that support such queries:

• [ ] Identify (e.g. by manually checking those in sources.json) or monitor for such a source.
• [ ] Add an example to the documentation of how to make such a query.

[ypldan]

Hi @khaeru, IMF_DATA & IMF_DATA3 provide support for such queries.

API documentation (requires auth, but new account can be easily created):

• SDMX 2.1
• SDMX 3.0

CC: @FedorYatsenko

[khaeru]

Great—thanks for the tip!