docs(hub): add Raspberry Pi (pre-built image) install flavor

[reneaaron]

Adds a new Alby Hub flavors → 🍓 Raspberry Pi page documenting how to run Alby Hub at home on a Raspberry Pi via a ready-to-flash image (no manual install): flash with Raspberry Pi Imager → boot → open albyhub.local.

• New page: hub/alby-hub-flavors/raspberry-pi.md
• Added to hub/SUMMARY.md nav (after Linux)

Notes / open items:

• The image isn't published yet, so the page is marked coming soon and links to getAlby/hub-os (the CustomPiOS build that produces the image). The OS-selection step and download/Imager-repo URL are placeholders pending the first release.
• Recommends Pi Zero 2 W (512MB) and up, USB-SSD boot for 24/7 reliability, and flags the Zero 2 W micro-USB / mini-HDMI connector gotchas.

Draft until the image ships and the download/Imager-repo link is final.

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