[docs] Render the RC Trial prerelease notices as caution callouts

Goal

Fix the prerelease notices on the English and Chinese RC Trial pages so they render as Starlight caution callouts instead of literal directive text.

Current state

These two files use a single-line :::caution[...] ... ::: form:

• apps/www/src/content/docs/en/start-here/rc-trial.mdx
• apps/www/src/content/docs/zh-cn/start-here/rc-trial.mdx

In the current production output, the directive markers and text are displayed as an ordinary paragraph. The nearby note component renders correctly, so the failure is isolated and easy to compare.

Suggested workflow

1. Change the directive markup to the syntax supported by the current Starlight/MDX pipeline.
2. Preserve the existing prerelease wording and inline version formatting.
3. Build and preview both locale pages.
4. Check the callout at narrow width and in light and dark themes.

AI-assisted syntax lookup and implementation are welcome. The rendered result must be inspected manually in the production preview.

Acceptance criteria

• The English prerelease notice renders as a caution callout.
• The Chinese prerelease notice renders as a caution callout.
• No literal :::caution marker is visible in the rendered pages.
• Existing wording, links, and version references are preserved.
• The callout remains readable at narrow width and in light and dark themes.
• corepack pnpm@10.32.1 --filter apps-www build passes.

Out of scope

• Rewriting the RC Trial guide.
• Changing release status or version promises.
• Redesigning all documentation callouts.

Maintainer notes

This is a Good First Issue because the two affected files, failure mode, expected rendering, and verification steps are fully specified.