docs: standardize sysfs paths on card0 for BC-250

The BC-250 enumerates as /sys/class/drm/card0 on a typical install (no
secondary GPU). Verified live on Fedora 43, kernel 7.0.9, PCI 1002:13fe.
Many doc snippets hardcoded card1, so copy-paste failed on a stock single-
GPU board.

Replaced /sys/class/drm/card1/ with /sys/class/drm/card0/ across:
bios/overclocking.md, bios/vram.md, drivers/radv.md,
getting-started/quick-start.md, linux/{arch,bazzite,cachyos,debian,fedora}.md,
reference/quick-reference.md, system/{governor,sensors}.md,
troubleshooting/{boot,display,performance,stability}.md.

Existing card0-vs-card1 disclaimers preserved at governor.md L166/L503,
quick-start.md L78, bazzite.md L104, kernel.md L222 (and quick-reference.md
intro rewritten to lead with card0 + note the iGPU edge case).

docs/troubleshooting/boot.md

@@ -760,7 +760,7 @@ xrandr --listproviders
 
 # DRM devices
 ls -la /sys/class/drm/
-# Should show card1, card1-DP-1
+# Should show card0, card0-DP-1
 
 # Current display mode
 xrandr

docs/troubleshooting/display.md

@@ -505,7 +505,7 @@ xrandr
 
 # Check DRM
 ls /sys/class/drm/
-# Should show card1, card1-DP-1, etc.
+# Should show card0, card0-DP-1, etc.
 ```
 
 ### Check GPU Initialization

docs/troubleshooting/performance.md

@@ -150,7 +150,7 @@ Apply to kernel source and recompile, or use a tool like `dkms` or CachyOS kerne
 
 **Verification:**
 ```bash
-cat /sys/class/drm/card1/device/pp_od_clk_voltage
+cat /sys/class/drm/card0/device/pp_od_clk_voltage
 # Should show range up to 2300MHz or higher
 ```
 

docs/troubleshooting/stability.md

@@ -648,7 +648,7 @@ watch -n 1 sensors
 
 **Terminal 2 - Frequencies and voltages**:
 ```bash
-watch -n 1 'cat /sys/class/drm/card1/device/pp_od_clk_voltage'
+watch -n 1 'cat /sys/class/drm/card0/device/pp_od_clk_voltage'
 ```
 
 **Terminal 3 - Memory**: