docs(cubie/a7z): fix EN UART debug page copy-paste from A7A

[tangzz-radxa]

Problem

The English UART debug page under cubie/a7z/system-config/ was copied from the Cubie A7A page and never fixed:

• The page title and body reference "Cubie A7A" instead of "Cubie A7Z"
• The connection diagram uses the A7A image (/en/img/cubie/a7a/a7a-uart.webp)
• The pin table labels use the A7A naming (UART0_TXD / UART0_RXD)
• The EN system-config/README.md also says "Radxa Cubie A7A system"

The Chinese page (docs/cubie/a7z/system-config/uart-debug.md) is already correct: it uses "Cubie A7Z", the a7z-uart-debug.webp image and UART0_TX/UART0_RX pin names. This PR aligns the English page with the already-correct Chinese version.

Fix

• i18n/en/.../cubie/a7z/system-config/uart-debug.md: "Cubie A7A" → "Cubie A7Z"; image → /en/img/cubie/a7z/a7z-uart-debug.webp; pin labels → UART0_TX (Pin 8) / UART0_RX (Pin 10)
• i18n/en/.../cubie/a7z/system-config/README.md: "Radxa Cubie A7A system" → "Radxa Cubie A7Z system"

No Chinese change is required — the Chinese counterpart already uses the correct content, so this PR aligns the English pages with it.

Scope

• docs-only, 2 files, single board section
• Discovered while reviewing issue Issue from cubie/a7z/system-config/uart-debug #1932 (user-reported UART device not detected; the copy-paste bug makes the EN instructions point at the wrong board)

Verification

• /en/img/cubie/a7z/a7z-uart-debug.webp returns 200 on docs.radxa.com (image already exists)
• No remaining "Cubie A7A" references in the A7Z EN section except the legitimate download.md product-family mention