New Swift-DocC content styling

[Karl]

@beatriz

I totally see your point. By default, we can have have a simple design like the one in these screenshots, though I appreciate your feedback— will continue to investigate whether the default options should be simpler. For example, just a solid color. Eventually, I'm excited for this to be a customizable component.

I had a quick go at prototyping it using the WebKit inspector. I appreciate you continuing to refine the default options, and customisation would definitely be welcome as well (I understand that's a bigger topic for DocC-Render).

@beatriz

The text below the page title continues to be the summary for that page. The "Inherited form View.cornerRadius(...)" is a bit of a confusing example, but that's actually content generated by Swift-DocC itself and something we'd like to improve there. For the common-case where you've written a documentation comment for that symbol, the symbol's summary would be rendered here. That's better represented in the other screenshots I shared above, for SlothCreator and Sloth.

Looking at some of the other pages from Ethan's argument parser demo, I wonder how well this scales. If the summary is small, things work relatively well. But this page, for example, has a 3-line summary.

At that point, you're basically reading the documentation of the symbol as centred text in the header, which doesn't feel very comfortable. Meanwhile, the content space below shows "No overview available".

That's quite unfortunate, because AFAIK there is no way to have the longer overview text without a summary.

I'm not sure if DocC-Render can be smarter about detecting these situations (and perhaps treat a long summary as an overview), or if the documentation just needs to be restructured to account for the new page style. In this case, I think a shorter summary is possible, but perhaps that is not always the case.

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Another thing I wonder about with these design changes is what the plan is for the table of contents ("On This Page"). Is the idea to remove it? Will it be replaced with anything?