docs: optimize Chinese RTD asset paths

[lkygithub]

Description

• Optimize raw HTML image references during Chinese locale-only Read the Docs builds.
• Derive the output-relative image prefix from each Markdown file's nesting depth, so the landing page and nested pages both retain valid asset URLs.
• Add a CI build that reproduces the zh-cn/latest Read the Docs layout and verifies the generated logo and architecture image references.

Motivation and Context

Read the Docs publishes the Chinese locale-only build at /zh-cn/<version>/. The Chinese landing page previously retained source-tree-relative raw HTML paths, causing its assets to resolve outside the version directory. Nested Markdown pages already resolve their image paths correctly, so the build-time adjustment is limited to matching raw HTML src attributes and accounts for each page's depth.

How has this been tested?

• mkdocs build --strict --config-file apps/rlark/mkdocs.yml for the normal bilingual site (73 HTML pages).
• Simulated the Read the Docs zh-cn/latest environment and ran a strict locale-only build (37 HTML pages).
• Verified the landing page emits images/logo-zh.png and images/architecture-zh.svg.
• Verified the architecture page retains ../images/architecture-zh.svg.
• Verified both generated asset files exist and repeated patch execution is idempotent.
• Validated the same candidate tree with an online Read the Docs test project.

Types of changes

• Documentation update
• CI coverage for the Chinese Read the Docs build
• Breaking change

Checklist

• My code follows the code style of this project.
• The Read the Docs build behavior is covered by CI assertions.
• All relevant new and existing documentation builds passed.