Create AI contibution guidelines

[gadomski]

I propose that we craft one based on GDAL's. I'm not sure if it'd be appropriate to apply the guidlines to all repos in stac-utils, I'll bring it up at the next STAC PSC meeting.

[gadomski]

I've opened a PR on the new https://stac-utils.github.io here: stac-utils/stac-utils.github.io#4