Add some minimal documentation regarding data import

[pReya]

The documentation works very well for new content. However it does not talk about importing existing data. Especially for the use cases, where a DB makes sense, I'd assume that many times some kind of legacy data needs to be imported. These questions are probably not unique to this database plugin, but similar when importing content into the file system.

• Is there a preferred/beneficial method for data import (e.g. direct DBMS/SQL import is not a good idea, because X/Y/Z – is it better to use the API for import purposes?)

• If one is using direct DB/SQL import: How can I generate a UUID? Is it just important that the IDs are distinctive, or do they need to adhere to a given algorithm/implementation?

• Could importing data from CSV/CLI be a good feature for the Kirby CLI?

• Maybe add a code snippet for programmatic import after the questions above habe been answered.