Document new post-import plugin preprocessing stage to amend settings

**Your Godot version:**

https://github.com/godotengine/godot/commit/6dc78c8aa17ad46e701e0c4e7b7632c2f0e098f1

**Issue description:**

Update the documentation.

> Allow post-import plugins to modify _subresources https://github.com/godotengine/godot/pull/100792

The use case is to write the .import manually. For example, take a glTF2 character and add HumanoidProfile remapping via script.

This was added as a last-minute advanced post-import plugin feature: post-import plugins can amend the `["_subresources"]["nodes"]["PATH:Armature/Skeleton3D"]["retarget/bone_map"]` settings from the preprocessing phase of a post import plugin

We can do it by writing a custom .import file, or we can do it with a plugin.

**URL to the documentation page (if already existing):**

https://docs.godotengine.org/en/stable/tutorials/plugins/editor/import_plugins.html