Add logo to HTML docs

[killerswan]

This adds the logo to the HTML versions of the docs, via CSS.

[brson]

This is a worthy idea but in my opinion the proposed style is not an aesthetic improvement (not that the existing style is attractive to begin with) because it adds too much useless whitespace. It also adds the rust logo to all subpages, so for example core::vec has a rust logo at the top. Additionally, any rust project that uses our style sheets will have a rust logo on their rustdocs.

For others' reference here's a screenshot: https://imgur.com/BJLkU

[killerswan]

I do think it is an improvement, and I did intend it to show up on the subpages.

Compare this with other projects' HTML documentation, which typically includes a logo and much more graphical customization:
http://nodejs.org/api/tls.html
http://clojure.org/Protocols
http://perldoc.perl.org/functions/crypt.html
http://docs.python.org/library/string.html
http://hackage.haskell.org/packages/archive/yesod/1.0.1.6/doc/html/Yesod.html

There are a lot of invasive changes we could make to more comprehensively style the pages, but I think just sticking a simple monochrome logo at the top goes a long way with minimal trouble. It will be trivial for us or other projects to replace or modify this small stylesheet later.

[graydon]

I think if you want to put an em-height logo at the right hand side of the main header or something (more like the size of the python and haskell logos embedded in their pages), that'd be ok. I agree with @brson though that the proposed change chews up too much blank space.

[killerswan]

How's this?

[brson]

That's kind of cute!

[brson]

Thanks for your patience!