Add MIDI loopback setup guide and link from README

[bseverns]

Motivation

• Provide a short, practical guide to create a real virtual MIDI port so TheMidiBus can open it without throwing a NullPointerException.
• Help users avoid the common "Real Time Sequencer" trap by documenting explicit setup steps for macOS and Windows.

Description

• Add a new doc docs/midi-loopback-setup.md with step-by-step instructions for enabling the IAC Bus on macOS and creating a port in loopMIDI on Windows.
• Link the new guide from the README.md under the "MIDI device gotchas" section for quick access.
• Keep the language concise and practical so users can quickly create a usable loopback port.

Testing

• This is a documentation-only change and no automated tests were run.
• Changes were committed to the repository and the new file docs/midi-loopback-setup.md was added successfully.

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Codex Task