Add a complete and verbose changelog

[dstansby]

As far as I can see, there is no complete changelog between versions of the OME-NGFF spec. The closest I can find is https://ngff.openmicroscopy.org/0.5/index.html#history, which only has a very short sentence on what changed in each version.

As a concrete example, in v0.5 axis types were narrowed so they have to be strings. But I have absolutely no easy way of knowing that this changed happened, making it very hard for me to make sure that I have finished updating an implementation of the v0.5 spec.

I propose that a complete, verbose changelog is added to the specification docs that explain every change that happens to the spec between versions. This would make it easy for users to understand what new features have been added, and easy for implementers to update their implementations with new versions of the spec.