iOS Plugin documentation

[naithar]

Related PRs for Godot: master godotengine/godot#41230, 3.2 godotengine/godot#41340
Proposal: godotengine/godot-proposals#1185

Adds a starting documentation for iOS plugin system implemented in linked PRs.
I've used Android plugin documentation as an example.

Header files could probably be made available just like Android's plugin .aar.

[Calinou]

The number of characters below a heading must match the heading above:

Suggested change

	iOS plugins
	===============
	iOS plugins
	===========

This is a Sphinx/reStructuredText requirement. Make sure to do this for all the headings you've added.

[NathanLovato]

Related to #4163

Thanks for the contribution. Would you mind if I directly edited the document to make the style fit our writing guidelines? https://docs.godotengine.org/en/latest/community/contributing/docs_writing_guidelines.html

You can check the guidelines and do it yourself if you'd like. I'm offering to edit for you because it's much more efficient than leaving you dozens of review comments.

[naithar]

Would you mind if I directly edited the document to make the style fit our writing guidelines?

If you think it's more efficient, sure, I wouldn't mind.

[NathanLovato]

I opened a new PR with some WIP edits. I hadn't noticed the feature wasn't merged in the engine; we should review and merge the docs only after a feature was added to Godot.

[naithar]

I opened a new PR with some WIP edits. I hadn't noticed the feature wasn't merged in the engine; we should review and merge the docs only after a feature was added to Godot.

Sorry, should have made it more clear in OP.

And it's also strange that you couldn't push directly to this PR as I have enabled edit by maintainers