docs: Add class-level Javadoc to API, platform and mixin classes; Add zh-CN docs

[FireOrchid2000]

This PR was generated with AI assistance. Please review carefully before merging.

What changed

Added concise class-level Javadoc comments (English) to 320 Java files across common, fabric and neoforge:

• Public API interfaces/classes under common/.../sable/api/
• Platform abstraction interfaces under common/.../sable/platform/ and core entry points (Sable, SableClient, config classes)
• All mixin classes in the three mixin packages — each comment states the mixin target class and its intent

What did NOT change

• No production code was modified: git diff shows 0 lines removed from non-comment code
• No file encodings were altered (verified no U+FFFD replacement characters)
• Comments are English-only, 1–3 lines per class, keeping {@link} references and existing formatting

Verification performed

• git diff: 320 files changed, +1104 insertions, 0 code deletions
• Scanned all touched directories for leftover CJK characters and encoding corruption: none
• Not compiled / not run with game tests — code review is requested

Notes for the maintainer

• Comments on mixins targeting MC internals or third-party mods (Create, Flywheel, Jade, etc.) were inferred from the @Mixin annotation, package path, and injected method names. These are the most likely to be inaccurate — please verify.
• No commit was made for this change set; the diff is ready to review in the working tree or a branch of your choosing.

[CLAassistant]

All committers have signed the CLA.