gitdatamodel documentation

@jvns wrote about this on her blog too: https://jvns.ca/blog/2026/01/08/a-data-model-for-git, which was posted here: https://lobste.rs/s/a2clkv/data_model_for_git. (Just in case anyone wants to hear more about the background and process.)

Thanks to the two authors for this work.

It's nice work but I always wonder why the engineers working on this couldn't be bothered to write something like this.