gh-114099: Add documentation for iOS platform

[freakboy3742]

Part of the PEP 730 work to add iOS support.

Adds an iOS usage guide (including documentation of how to add iOS to your own Xcode project), and flags modules in the standard library that aren't available on iOS.

Fixes #115977.

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📚 Documentation preview 📚: https://cpython-previews--117057.org.readthedocs.build/

• Issue: gh-114099

[freakboy3742]

The linting check is currently picking up a missing link to platform.ios_ver(); this method is added (and documented) in #117052.