document .gitignore. Add .DS_Store to it while at it.

[JacquesCarette]

As per title.

[JacquesCarette]

So it seems to pre-commit hook didn't like my changes to .gitignore. Are the rules to obey written down somewhere?

[wenkokke]

The pre-commit setup sorts it alphabetically, which is incorrect given that .gitignore exclusions and inclusions are order-dependent, so that should probably be taken out.

(The details are in the pre-commit config file. Follow the links to the various repositories to find out what code each action runs.)

[JacquesCarette]

This one is ready for review @wenkokke .

[wenkokke]

I meant after their respective inclusions, eg, the zip inclusion after the general exclusion, but that's alright, we can fix that later.