docs(cubie): clarify A7Z GPIO overlay examples

[tangzz-radxa]

Summary

• add a small overlay note to the Cubie A7Z 40-pin GPIO page
• explain that rsetup only exposes common overlay examples instead of every possible pinmux combination
• point users to the GPIO multiplexing table, Install 3rd party overlay, and the radxa-overlays examples when they need other pins
• keep the English page in sync with the Chinese source

Why

Issue #1214 shows that readers can misread the current GPIO page as if the few built-in rsetup overlay entries are the only supported routing options. A small docs-only clarification is enough here: the board exposes more pinmux choices, while rsetup ships a practical subset of examples.

Validation

• ./scripts/agent-doc-lint.sh docs/cubie/a7z/hardware-use/pin-gpio.md i18n/en/docusaurus-plugin-content-docs/current/cubie/a7z/hardware-use/pin-gpio.md
• ./scripts/agent-doc-translation-guard.sh
• git diff --check

Fixes #1214