Add a pre-commit hook that automatically checks if GES children properties
documentation is up to date when GES source files are modified. The hook
runs the document-children-props.py script and warns if the generated
documentation files have changes that need to be committed.
This ensures that children properties documentation stays synchronized
with code changes and prevents outdated documentation from being merged.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5987>
3fe0127a64