Detect new, changed, or removed features

This feature should probably be part of the docs merge tool.

It'd be nice to have some metadata to:

- Display the version of the project in the generated markdown. We can get that from `project.godot`. See #36, it needs to be done first
- Detect new, modified, or removed features in a given update. That would happen comparing the version numbers and/or changes upon merging the data.
- We can then add a metadata field to each relevant json object to add a [configurable] new html tag and css class name to the entries in the generated markdown

See `merge_json.py` to write that code.