docs(i18n): add complete Simplified Chinese documentation

[old-Jerry]

Summary

This draft adds a complete Simplified Chinese translation of the Red Pitaya documentation, based on upstream revision 2bb2c8e3bfae2c99653348c5815c8c8adaab9a66 (Read the Docs latest build 33959913).

• Adds a parallel, buildable Sphinx source tree under zh_CN/
• Preserves upstream paths, labels, references, includes, images, downloads, toctrees, code blocks, commands, identifiers, and URLs
• Translates all 378 upstream RST pages
• Includes terminology, progress, revision, reuse workflow, changelog, and validation reports under translations/zh_CN/
• Prioritizes and reviews STEMlab 125-10, hardware, acquisition/generation, streaming, SCPI/API, and current FPGA learning paths

Validation

• Translation inventory: 378/378 pages complete
• Missing Chinese pages: 0
• Pages identical to English source: 0
• Suspected untranslated prose: 0
• Structural differences: 0
• New duplicate labels introduced by Chinese sources: 0
• Strict Sphinx Dummy build: passed with -E -W --keep-going
• Strict Sphinx HTML build: passed with -E -W --keep-going
• Bounded link check: completed to its 180-second limit; inherited upstream 404/403, certificate, redirect, and timeout results are recorded without changing official link targets

Maintainer decision requested

The draft currently uses a complete parallel Chinese RST tree. Before this is marked ready for review, we would appreciate guidance on the preferred long-term localization architecture:

1. Keep the parallel zh_CN/ RST tree in this repository.
2. Convert the reviewed translation into Sphinx gettext/PO catalogs.
3. Maintain Chinese in a separate repository or branch and register it as a Read the Docs translation project.

The existing Chinese RST tree can serve as the reviewed source for a later automated migration to PO catalogs, so no content needs to be translated again.

We would also appreciate confirmation of the documentation licensing and contribution policy, because the locked upstream revision does not contain a root LICENSE, COPYING, CONTRIBUTING, or pull request template.

Review notes

This is intentionally opened as a draft because the integration and Read the Docs language-switching approach require maintainer direction. The translation content and builds are complete; the remaining work is choosing the upstream maintenance model and applying any requested repository layout changes.