rustdoc: design fixes and changes

- Closes #13591. Relevant example: http://adrientetar.legtux.org/cached/rust-docs/struct.CChars.htm
(Had to use `!important` to override CSS selector precedence, namely matching over parent class.)
- Implement changes from #13780 feedback, namely:
  * Changed font-size from 18px to 15px
  * Reintroduced gray background for code samples
  * Tightened up the margins
- Fix point 1 and point 4 of #13804.

Samples:

- [enum.FileType](http://adrientetar.legtux.org/cached/rust-docs/enum.FileType.htm)
- [struct.CChars](http://adrientetar.legtux.org/cached/rust-docs/struct.CChars.htm)
- [std](http://adrientetar.legtux.org/cached/rust-docs/std.htm)
- [std::io](http://adrientetar.legtux.org/cached/rust-docs/io.htm).

r? @brson