Add `aiida` as a `verdi` alias (for now?)

[edan-bainglass]

AiiDA should be represented by aiida at the command line. Seems obvious. Appears to have been the initial choice but gave way to verdi due to a conflict with the aiida python package itself. However, this appears to no longer be the case. See https://github.com/orgs/aiidateam/discussions/6381.

Of course, if agreed/accepted, we should consider how to proceed with the docs. At least three votes so far in favor of aiida going forward, verdi as legacy, so reflect this in docs.