the "change scenes manually" page should be moved to after the "using SceneTree" page

**Your Godot version: 4.6**

**Issue description:**
These 2 pages (mentioned in the title of this issue) in the "Core features" chapter are in a confusing order in my opinion.

Indeed, the "change scenes manually" page:
- mentions autoloads (introduced in a [later page](https://docs.godotengine.org/en/stable/tutorials/scripting/singletons_autoload.html))
- says "Sometimes it helps to have more control over how you swap scenes around"
- which feels like it belongs after the ["Changing current scene" paragraph](https://docs.godotengine.org/en/stable/tutorials/scripting/scene_tree.html#changing-current-scene) of the "using SceneTree" page.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/tutorials/scripting/change_scenes_manually.html
https://docs.godotengine.org/en/stable/tutorials/scripting/scene_tree.html
https://docs.godotengine.org/en/stable/tutorials/scripting/index.html#core-features