docs(accessories): note that the Metal Case for ROCK 5B does not fit ROCK 5B+

[tzz-radxa]

Summary

• add a note to the Metal Case for ROCK 5B page clarifying that this case is for ROCK 5B only
• explain that the ROCK 5B+ has its SD card slot in a different position, so the ROCK 5B case does not fit it
• point ROCK 5B+ users to the dedicated Radxa Metal Case for ROCK 5B+
• keep Chinese and English pages in sync

Why

A user reported on the docs discussion for this page that the Radxa Metal Case does not fit the ROCK 5B+ because the SD card slot is in a different position, and asked whether any case exists for the ROCK 5B+. The docs page only listed ROCK 5B compatibility and never mentioned the ROCK 5B+ case, which made it look like no case was available. This note removes that ambiguity at the point where users land.

Verification

• Chinese page: docs/accessories/heatsink-case/metal-case-rock5b.md updated
• English page: i18n/en/docusaurus-plugin-content-docs/current/accessories/heatsink-case/metal-case-rock5b.md updated
• both files verified with the docs PR guard (github_pr_guard.py check): single scope, bilingual OK
• ROCK 5B+ case reference checked against the public product page: https://radxa.com/products/accessories/metal-case-for-rock5bp

Related discussion: #2123