Remove csv.gz from ehrql tutorials and how-tos

[bloodearnest]

Currently, the getting started and many other examples use ehrql, but output csv.gz files rather than arrow.

csv.gz is a very expensive format CPU wise on the server, so we want users to use arrow where ever possible, and so we should update the docs to use arrow outputs.

This may need to be done in co-ordination with opensafely-core/ehrql#2092

[bloodearnest]

It may be that some of this needs to be done in the ehrql repo?

[bloodearnest]

We may also want to mention viewing arrow files locally with a vscode extension, depending on opensafely/research-template#56 (comment)

[lucyb]

I'm going to use opensafely/research-template#56 as the "epic" to track this work. Not least because the Getting Started Guide will need to be updated at the same time as the research template. However, we don't have capacity in Team REX to start on it immediately, so I'm going to remove this from the board for now.

If there are areas of the ehrql documentation that can be updated now, without further changes being made, then they should be captured in separate tickets and assigned to Team RAP.