Update WIT Reference with new types from WIT Spec 2026-07

As of writing, `map` types, nested package defintiions, and `@external-id("example.com")` have been added to the official [WIT spec](https://github.com/WebAssembly/component-model/blob/main/design/mvp/WIT.md).

The [component-docs reference](https://component-model.bytecodealliance.org/design/wit.html) should be updated (for at least the map type) to reflect the changes.

Compare the component-docs WIT reference at https://component-model.bytecodealliance.org/design/wit.html with the official WIT spec linked in the issue. Update the reference to cover the new map type, nested package definitions, and @external-id("example.com") where applicable; done means the documentation reflects the cited spec changes.

索引モデルが issue の本文から書いたものです。