Add structured data for the home page

[kevmoo]

See https://search.google.com/structured-data/testing-tool/u/0/?url=https://pub.dartlang.org/#url=https%3A%2F%2Fpub.dartlang.org%2F

VS

https://search.google.com/structured-data/testing-tool/u/0/?url=https://pub.dartlang.org/#url=https%3A%2F%2Fpub.dartlang.org%2F
Should enable

https://developers.google.com/search/docs/data-types/sitelinks-searchbox
https://developers.google.com/search/docs/data-types/logo
https://developers.google.com/search/docs/data-types/social-profile

[kwalrath]

I think the second URL should be https://search.google.com/structured-data/testing-tool/u/0/?url=https://www.dartlang.org/#url=https%3A%2F%2Fwww.dartlang.org%2F.

More info:

• https://developers.google.com/search/docs/guides/prototype
• https://developers.google.com/search/docs/guides/search-gallery

@kevmoo, do you have any particular recommendations for how we should add structured data?

[kevmoo]

We put it in as ld+json. See

https://github.com/dart-lang/pub-dartlang-dart/blob/1ca93dcb1d1e1489bec4768de553b9091974cc58/app/test/frontend/golden/index_page.html#L115

I could imagine some clever scripting to do this for all pages, but AFAIK just getting the homepage bits in would be a help...

[kwalrath]

We already have a search box and logo in our results, even without this:

But no social stuff, perhaps because we don't prominently link to it. Perhaps we should fix #256 first, and then see if we get social listings as a result.

Are there other reasons to add this structured data?