Tracking: WASI P3 documentation updates

[ericgregory]

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<T>, future<T>) 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