Add Raspberry Pi headless configuration notes

Document bootfs headless setup: userconf.txt password hash, and
discovery/login steps via nmap and ssh.

LLM-Generated: true

active/device_raspberry_pi/raspberry_pi.md

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+# Raspberry Pi
+
+## Headless Configuration
+
+1. Create a file called `ssh` in `bootfs`
+2. Create a file called `userconf.txt` in `bootfs`
+3. Run `echo 'YourPassword' | openssl passwd -6 -stdin` in your terminal to generate your encrypted password
+4. Edit `userconf.txt` and add a single line: `username:encrypted_password`
+5. Save `userconf.txt` and eject the SD card
+6. Plug in the pi
+7. Find the pi with `nmap -sn 10.0.0.0/24`
+8. Login with `ssh username@pi-ip-address`