docs(orion/o6): point cix-sdk tutorial to official CIX wiki

[tangzz-radxa]

Summary

Replace the locally-maintained CIX SDK build / source-acquisition tutorials under orion/o6 with a pointer to the official CIX documentation, since the build flow is now maintained upstream at https://github.com/cixtech/cix-manifest/wiki.

Changes

• docs/orion/o6/low-level-dev/cix-sdk/build.md (zh)
• docs/orion/o6/low-level-dev/cix-sdk/get-source.md (zh)
• docs/orion/o6/low-level-dev/cix-sdk/README.md (zh)
• i18n/en/.../orion/o6/low-level-dev/cix-sdk/build.md
• i18n/en/.../orion/o6/low-level-dev/cix-sdk/get-source.md
• i18n/en/.../orion/o6/low-level-dev/cix-sdk/README.md

Both build.md and get-source.md are converted from doc_kind: wrapper (importing from the shared orion-common partial) to doc_kind: page with a single info callout pointing to the CIX wiki:

• 中文：https://github.com/cixtech/cix-manifest/wiki/guide_zh
• English: https://github.com/cixtech/cix-manifest/wiki

The README.md index adds the same pointer at the top, and still renders <DocCardList /> so the existing sub-pages remain reachable.

Scope notes

• The shared docs/common/orion-common/low-level-dev/cix-sdk/_build.mdx and _get-source.mdx are intentionally not modified in this commit. orion/o6n still imports from them, so O6N continues to render the old tutorial. If O6N should be redirected too, please comment and I'll send a follow-up commit on the same branch.
• The O6N sync is intentionally deferred so that this PR is scoped to the explicit ask (O6 only) and avoids a stealth change to O6N.

Verification

• scripts/agent-doc-lint.sh → passed
• scripts/agent-doc-translation-guard.sh → passed
• github_pr_guard.py check → ok (1 commit, 6 files, bilingual complete)