Surface category one liner on index page

[parlough]

Surface the first line of category documentation on the documentation index page.

Contributes to #3618, by enabling the first line a category/topic's documentation to show up on the index page. If a category does not have a documentation page (such as the Dart SDK's categories), this doesn't change anything.

Example from test_package index page:

[srawlins]

This is great! Can you paste what this does to the Dart SDK docs (which I think use categories for "web", "VM", etc.)?

[parlough]

I've updated the PR description to expand a bit, but this won't affect those categories yet.

As far as I understand, a category with documentation is treated like a topic, but I don't know if we want to treat those categories as topics in the SDK docs. So some mechanism needs to be introduced that prevents a category with documentation being treated as topic, while still having documentation. I haven't determined the best path forward yet, so am open to suggestions!

I thought landing this would be a good first step, as it benefits categories that already are topics. Then hopefully we can build on this for simpler categories like in the SDK :)

[srawlins]

OK, so this change would not add any text to the api.dart.dev home page?

[parlough]

OK, so this change would not add any text to the api.dart.dev home page?

Correct. As those categories don't have any descriptions.