Document after-pull upgrade steps for developers

[EdNutting]

We should create a short developer guide document that summarises common steps to take when pulling updates. For example, when to rerun GQL Codegen (almost always after pulling); when to run Hasura migrate/apply metadata (whenever the "database changes" signifier / emoji is in a pulled commit); when to reinstall node packages, etc

We should also document requirements like node versions and such, as well as common tools we use in our dev setups.