Tracking: WASI P3 documentation updates

The Component Model book needs to be updated for P3. This issue tracks the overall effort; individual child issues will be filed for each reviewable unit of work below.

The high-level approach is inline tabbed content for sections that differ between P2 and P3. Pages that are version-agnostic stay untouched. A small admonition at the top of each version-variable page signals that tabbed content is present.

Infrastructure landed in #340:

- `mdbook-tabs` preprocessor registered with theme-aware `tabs.css`
- `version-notice.css` for the page-level callout
- `/running-components/wasmtime.html` converted as the first real demonstration

Remaining work (child issues proposed for filing after discussion):

- [ ] Document WASI P3 async primitives (`async func`, `stream`, `future`) in the Understanding section
- [ ] Update WIT reference and WIT by Example for WASI P3 syntax
- [ ] Convert Rust "Building a simple component" guide for WASI P3
- [ ] Convert JavaScript "Building a simple component" guide for WASI P3
- [ ] Convert jco "Running components" page for WASI P3
- [ ] Add a WASI P2 to P3 migration guide
- [ ] Update the Canonical ABI page with async extensions
- [ ] Update the FAQ with WASI P3 questions
- [ ] Cross-link with WASI.dev for WASI P3 content

Related: #340

先阅读 #340 以及其中列出的 P3 工作领域：Understanding、WIT reference、WIT by Example、Rust 和 JavaScript 指南、jco 的 running-components 页面、Canonical ABI、FAQ 和迁移内容。在选择一个单元之前，确认哪个 child issue 可用；完成意味着提交一个可供审查的 P3 文档更新，其中包含适当的版本化选项卡、notice 和交叉链接。

由索引模型根据 Issue 内容生成。