doc: Remove h[123] border and increase their padding to better readability

After reading issue #7077, all header elements had a border. In my opinion those borders are a bit too much distraction. I tried a different approach with increasing the padding and font size, and omitting the borders.

Comparison:
http://smvv.io/rust-doc/std/hashmap.html
http://static.rust-lang.org/doc/std/hashmap.html

Note: the highlighted code blocks are not caused by this commit.

I left the border of the code block / function signature as is. The reason behind that is that code blocks are really block elements, while headers are not. What do you guys think?