Add ios-cmake toolchain docs

[a4z]

[CLAassistant]

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[lasote]

Thanks for this. I think first we need to try supporting all this into the new CMakeToolchain to have native support in Conan.

[a4z]

Not sure what you mean, ios-cmake is independent, you can use it basically stand alone, with out anything else, and it will just work

[lasote]

Then probably shouldn't be documented in the conan docs. Or not like this, I meant we should explore a better integration with the toolchain than using a package with a build require. That's all.

[a4z]

Then explain why the so called conan-darwin-toolchain is mentioned. It was a point of confusion for many people over the last years. They got through the conan docs to that package, instead to the ios-cmake toolchain, I fix that here. The way to build for iOS tvOS watchOS is ios-cmake, that is proven to work for many people since years.

[lasote]

Because the project evolves. Nowadays I wouldn't add conan-darwin-toolchain either.