Include `__array_api_version__` in API specification section

[honno]

xp.__array_api_version__ has been specified in our Versioning section for a while now (~2yrs), but no such mention of it is found in the API specification section, which is probably why this wasn't implemented in numpy/numpy/pull/18585 and all subsequent adoption efforts across the ecosystem (happy to submit respective PRs after we get this in).

So this PR simply creates a spec/API_specification/version.rst file which includes a documented __array_api_version__ attribute, which also will show up in our array-api/latest/API_specification/ page (as well as the sidebar).

[Zac-HD]

After this is merged, let's also add a test to the conformance suite.

[leofang]

Merging as this got two green checks and is what we should have had.