iOS plugin documentation for the 3.2 branch

[mbrlabs]

Your Godot version:
3.2.4 rc4

Issue description:
I think all the documentation PRs (#4213 and #4693) dealing with the new iOS plugin system targeted master and not 3.2 and have not been cherry-picked. I don't know what the workflow here is, but this should probably be included in the 3.2 documentation branch before 3.2.4 ships ;)

URL to the documentation page:
See the linked PRs

[skyace65]

Closed by #4737.