[FEATURE] Add documentation for animation file format

Currently, there's no documentation about the animation file format.

It would be useful to add some info about, for example, the changes between each file format version.

The information should be mostly retrieved from repo history, which may prove tedious.

Start by reviewing the repository history for the animation file format and identify each version change. Document the format and the differences between versions, with completion marked by a clear reference that covers the historical changes.

Written by the indexing model from the issue text.