remove horizontal rule from Markdown-generated HTML

[jsha]

When documentation has markdown headings (e.g. # Foo, ## Bar, etc), we currently display those with a horizontal rule (<hr>) across the whole page. However, that makes sub-sub-headings, like Panics in the below screenshot of https://docs.rs/rctree/0.4.0/rctree/struct.Node.html, look more prominent than their parents in the document structure. They also make the page look grouped along lines that it's not intended to be grouped along.

We should remove or hide these elements to improve the scannability of pages. Most likely we want to keep them when they are part of the docblock at the very top of the page, but hide them elsewhere.