Replace most leftover "instance" verbs with "instantiate"

[Mickeon]

In Godot 4.0, in a moment of lucidity, PackedScene's instance() function was renamed to instantiate() and the surrounding documentation followed suite. Despite this, there's some leftover text, or even new text, that didn't quite get the memo.

This PR replaces every occurrence of "instance" being used as a verb with "instantiate".
Similarly, "instancing" has been replaced with "instantiation" where appropriate.
File names are not changed in this PR.

Keep very strongly in mind the following:

• Instantiation is the whole creation process.
• Instancing is a real term in graphics rendering. It refers to the grouping of similar objects to render them all in one swoop. It's not the same concept as instantiation.
  • Similarly, an object is said to be instanced when it is, or can be, part of this batch, but this meaning very rare in the docs.