Add remark about owner to the instancing tutorials and link scripting with editor tutorial

Following [this comment](https://www.reddit.com/r/godot/comments/esbtzd/how_to_navigate_the_scene_tree_20_by_theduriel/ff9sel9?utm_source=share&utm_medium=web2x) by @willnationsdev , it seems like [this issue ](https://github.com/godotengine/godot-proposals/issues/390) could improved by just adding
`node.owner = self`
when instancing a node via code.

My request is to make a remark [here](https://docs.godotengine.org/en/3.1/getting_started/step_by_step/scripting_continued.html#instancing-scenes), that it is necessary to add `node.owner = self` when instancing via code, otherwise the instance will have no owner. Also making a remark that instancing in the Editor automatically assigns an owner.

My other request is to also have a instancing script sample [here](https://docs.godotengine.org/en/3.1/getting_started/step_by_step/instancing.html), or at the very least link to the script tutorial with sample [here](https://docs.godotengine.org/en/3.1/getting_started/step_by_step/scripting_continued.html#instancing-scenes), because when online searching for "Godot instance" this is the page that comes up first, but it has no coding examples what so ever. In the beginning I thought there is just no example script.