docs: document test fixture generation

[alestiago]

Description

As a maintainer I would like to have proper documentation explaining how to generate test fixtures.

Additional context
Test fixtures such as this one are unintuitively not a plain snapshot (as explained here) of the pub.dev packages response.

[marcossevilla]

@alestiago what's the process here so we can write these docs?

[alestiago]

Hi @marcossevilla great question. Looking at the blame it seems that @felangel was the first one to generate the file.

I'm not sure if the entire file was generated using a tool, or if instead it was manual. It might be that the response from the mentioned endpoint was manually copy pasted into the Dart variable, and other content such as the header and the other variable were manually written.

[marcossevilla]

hey @felangel, do you remember what was the process here?