Update: Skip processing substitution_definition nodes

[Andrej730]

And leaving them as-is is producing errors later in format_validator transformer.
Resolved blocking issue from #399

[nutti]

Please do this in the transformer.
https://github.com/nutti/fake-bpy-module/blob/main/src/fake_bpy_module/transformer/rst_specific_node_cleaner.py

This should be CodeDocumentNode to remain its contents as a comment.

[Andrej730]

@nutti moved handling to the transformer. But couldn't get past the line endings restrictions to commit my changes without applying line endings normalization first, see ##409