Add convenience function for defining expected Regions and Units on ixmp4 Platforms

### What is this about?

In order to use an ixmp4 `Platform` with message_ix, we need to define various Regions and Units on that Platform. In prior discussions, @danielhuppmann, @phackstock, and @meksor suggested that the best way to do that might be to expose a function like `message-ix setup-db ` via the CLI that adds a standard set of units as well as regions to a Platform. The regions could potentially even be specific to "R11", "R12", etc. This function would be required to be called every time one sets up a DB, which is a change to existing workflows, though not a big one as each DB only needs to be set up once.
In the tutorials, we would already teach users that this needs to happen and explicitly define the required units to show how to do it manually.

Start by reviewing the existing CLI setup flow and the tutorials that manually define Regions and Units. Clarify the standard units, supported region codes such as R11 and R12, and how the proposed `message-ix setup-db ` command should behave; done means a documented, agreed setup workflow for ixmp4 Platforms.

Written by the indexing model from the issue text.