docs: add Dragon Q8B 40-pin GPIO guide

[tzz-radxa]

Summary

• add Chinese and English Dragon Q8B 40-pin GPIO guide pages
• reproduce the supplied 40-pin multiplexing table in the existing Dragon Q6A table style
• reuse the existing Q8B GPIO interface diagram and add safe GPIO controller discovery steps
• place the guide between HDMI and M.2 E Key in the hardware-use navigation

Validation

• scripts/agent-doc-lint.sh passed for all changed files
• scripts/agent-doc-translation-guard.sh passed
• verified all 20 table rows and 40 pins against the supplied data
• github_pr_guard.py check passed with one commit, four files, one scope, and complete zh/en coverage

Note

The usage section intentionally does not hard-code a /dev/gpiochipN device or line offset. Those values can vary by system image and kernel version, so the guide instructs users to verify them with gpiodetect and gpioinfo first.