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