Document `@docImport` in `README.md`

[jonasfj]

Implemented in https://github.com/dart-lang/sdk/issues/56186 (credits @@srawlins).

We should probably document this on dart.dev too. But I figured it would be harmless to add it to dartdoc. This is certainly one of the placed I went to look for it :rofl:

[srawlins]

Thanks for the PR! I'd like to put this off until the implementation is a little bit tidied. There are a few items I hope to finish this quarter before "releasing" the feature:

• prefixes (@docImport 'foo' as foo;)
• reporting unused doc-imports
• reporting imports that could be doc-imports

[FMorschel]

@srawlins, what about this PR now that we have https://dart.dev/tools/doc-comments/references#doc-imports?

[srawlins]

Oh wow, blast from the past 😅 yes I'll review this anew.