docs: clarify macOS PyQt embedding

[huhaha120]

Summary

• clarify the macOS/PyQt workaround for issue Stuck in threading on macOS #61: create the Qt window on the main thread and pass its wid to mpv
• keep the embedded MPV instance alive as self.player for the lifetime of the window
• remove the Linux-specific vo='x11' option from the PyQt example so it is less misleading for macOS users
• fix the "Issue 102" README link, which currently points to issue Stuck in threading on macOS #61

Verification

• git diff --check
• README content checks:
  • issue Cannot enable pseudo GUI mode #102 link now points at /issues/102
  • issue Stuck in threading on macOS #61 remains linked in the platform notes
  • the PyQt section documents the macOS main-thread constraint
  • the PyQt sample uses self.player
  • the sample no longer contains vo='x11'

Note: this is a documentation-only change. I could not render the RST locally because docutils is not installed in this environment.

Refs #61.