Document PQS values for Assets Importing in the docs

[marthakeezy]

At the moment, when the user goes to import Assets, they can run into errors where the cell is filled out incorrectly, but the user does not know what values they can enter to begin with.

For example, what is a valid Category or what valid Types are out there? Referencing the discussion in Product: iSC - CCEI management module chat, we need to document the values somewhere in our docs to make the data validation easier and to help users navigate the importing.

Ideally would be documented here: https://docs.msupply.foundation/docs/catalogue/assets/