Inflated sidebar content for large packages

[InMatrix]

@timsneath originally reported the issue in this comment:

Related, I wonder if we can review the content in the sidebar. It's actually not very helpful, if you look at a typical page in the Win32 package. And that's not isolated to my package: it's hugely inflated even for a Flutter library. I'd be surprised if it was used by most customers of large-scale packages.

[devoncarew]

FYI, this PR:

#3018

Removes the sidebar for all elements which do not themselves contain members. So, it retains it for things like libraries, classes, and extensions, but removes it for things which are essentially leaf nodes of the API docs.