5.23.0: docs: add iOS SDK build and integration instructions (fixes #769) (#864)

- Add dedicated iOS section in docs/BuildBrainFlow.rst with SPM, XCFramework, permissions, and compilation guides
- Add iOS cross-reference to the Swift section in BuildBrainFlow.rst
- Add iOS to supported platforms for Synthetic, Streaming, and Playback boards in docs/SupportedBoards.rst