How do these external identifiers work, and how do I get something into one of these namespaces? (I apologize if I have missed them in the documentation)
If I stand up my own wikibase with the Docker containers and create an item for the Mayor of Madison, WI - lets say that creates Q2 in my local wikibase, and it will be accessible via
http://localhost:8181/wiki/Item:Q2
Is there some way I could create an item in my local wikibase that would have a URL of
http://localhost:8181/wiki/Item:wd:Q16107138 that represents Paul Soglin back in Wikidata but stored in my local wikibase, and that I can reference in other properties in my wikibase - if I create my own entry Q3 for Madison, it'd be nice to be able to point 'head of government' to wd:Q16107138 and be able to use it in my local wikibase and mediawiki instance, so if I have a page for Madison as well as a wikibase entry in the local install I can deference it in the wiki markup.
Or are those namespace identifiers like wd: (and xy: or foo: or whatever namespace) only in the WDQS for making calls out to SERVICE bits in SPARQL, plus whatever the WDQS exporter generates for local RDF?
(Also my apologies if wikibase doesn't work like this at all and I've so badly interpreted what Daniel is saying that I'm about to throw the whole conversation in a dead-end direction)
Thanks!